Imported: 10 Mar '17 | Published: 27 Nov '08
USPTO - Utility Patents
A transfer drum for conveying a sheet in a machine of the graphic arts industry. The transfer drum is developed for controlling grippers without much effort and without much wear to the components of the gearing mechanism. The transfer drum includes grippers that are suited for holding an edge of the sheet and are arranged on a gripper shaft. Levers are fixed to the ends of the shafts. A cam mechanism for opening and closing the grippers has control cams that are connected to the frame and have contact surfaces, and cam followers fixed to the levers and in contact with the contact surfaces. Opposing torques act on the ends of the gripper shaft.
This application claims the priority, under 35 U.S.C. 119, of German application DE 10 2007 024 646.5, filed May 24, 2007; the prior application is herewith incorporated by reference in its entirety.
The invention relates to a transfer drum for conveying a sheet in a machine of the graphic arts industry. The drum has a drum body supported in a frame rotatably about an axis. A gripper shaft extends parallel to the axis of the drum body. Grippers are mounted on the gripper shaft for holding an edge of the sheet. Levers are fixed to the ends of the gripper shaft, and a cam mechanism is disposed for opening and closing the grippers. The cam mechanism includes control cams that are connected to the frame and have contact surfaces, and cam followers that are fixed to the levers and are in contact with the contact surfaces.
Transfer drums with gripper systems for holding an edge of a sheet are used for conveying sheets through a printing press. A gripper system includes a plurality of gripper fingers, which are supported on a gripper shaft and cooperate with other gripper fingers or gripper pads. In order to open or close the grippers, the gripper shaft is supported on the drum body so as to pivot about an axis that is parallel to the axis of rotation of the transfer drum. The pivoting movement is controlled by a gearing mechanism, in particular a cam mechanism. When a cam mechanism is used, the gripper shaft is connected to a roller lever. A cam roller supported on the roller lever interacts with a cam that is fixed to the frame of the machine. To ensure that a printed image has a desired position on the sheet, the grippers need to hold the edge of the sheet with reproducible accuracy. The gearing mechanisms for controlling the grippers are designed in such a way that the grippers carry out a functional movement when the transfer drum is in an angular position in which a sheet is to be transferred to the grippers. In the case of cam gearings, the cam follower must contact the contact surface of the cam when the transfer drum is in the aforementioned angular positions.
Commonly assigned German published patent application DE 102 48 974 A1 describes a gripper arrangement with a cam roller that is held in friction-locked engagement with a control cam by means of a torsion bar spring. The torsion bar spring is arranged inside a gripper shaft. The ends of the torsion bar spring are fixed against rotation on a roller lever of the cam follower and on a tensioning lever. The tensioning lever is used for the dynamic adjustment of the engaging force between the cam follower and the control cam.
U.S. Pat. No. 5,813,338 and its corresponding German published patent document DE 697 08 026 T2 disclose a mechanism for opening and closing grippers of a sheet transport cylinder. The mechanism includes cam followers that are held in form-fitting engagement with contact surfaces of control cams. Double levers with cam followers that cooperate with double cams are arranged on the ends of a gripper shaft. The cams of a double cam are twisted with respect to each other in such a way that the cam rollers rest on the contact surfaces without play.
If a torque is applied to only one side of a gripper shaft to open and close grippers, an identical opening and closing instants must be set for the grippers due to the torsion of the gripper shaft. To reduce the effects of a torsion of the gripper shaft, German published patent application DE 35 08 697 A1 and its counterpart U.S. Pat. No. 4,759,290 describe a device in which the torque is introduced at the center of the gripper shaft. Additional gearing elements are required to be able to introduce the torques at the center.
It is accordingly an object of the invention to provide a transfer drum that is suitable for conveying a sheet in a machine of the graphic arts industry, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows to control grippers without much effort and without exposing gearing elements to much wear.
With the foregoing and other objects in view there is provided, in accordance with the invention, a transfer drum for conveying a sheet in a machine of the graphic arts industry, comprising:
a drum body rotatably mounted in a frame for rotation about a drum axis;
a gripper shaft disposed parallel to said drum axis, said gripper shaft having ends;
grippers mounted on said gripper shaft and configured for holding an edge of the sheet;
levers affixed to said ends of said gripper shaft;
a cam mechanism for opening and closing said grippers, said cam mechanism including control cams connected to the frame and having contact surfaces, and cam followers affixed to said levers and in contact with said contact surfaces; and
wherein said ends of said gripper shaft are subjected to mutually opposing torques.
In other words, the objects of the invention are achieved in that cam mechanisms with stationary cams whose follower elements are fixed to the ends of a gripper shaft are used to open and close grippers on a transfer drum. The follower elements, in particular engagement elements consisting of a lever and a cam roller, are fixed to the gripper shaft in such a way that torques of opposing directions are applied. The cams are preferably formed identically, i.e., they follow the same cam race design.
In lithographic offset presses in which one printed image is created on a sheet with every revolution of a plate cylinder, a gripper system may be provided on precisely one gripper shaft on a transfer drum. Furthermore, it is possible to provide multiple gripper systems and gripper shafts on a transfer drum. For example, a transfer drum may be driven at half the angular speed of a plate cylinder and may have two gripper shafts that are arranged diametrically opposite each other. At the ends of each gripper shaft, a lever and a cam roller are provided, and the cam rollers rest on common shaft in a force-fitting engagement.
Transfer drums include all cylinders and drums that convey sheets such as impression cylinders, transfer drums, storage drums, reversing drums. In accordance with the invention, each gripper system is actuated on both sides with opposing torques so that the occurring torsion is reduced compared to the solutions of the prior art. The engagement elements, in particular the cam rollers, are guided in a force-fitting way on the contact surfaces, such that positive contact is ensured and the system is not sensitive to manufacturing and assembly tolerances. Due to this positive contact, no undesirable spring action is required. Compared to a one-sided actuation, the load applied to the cam mechanisms is reduced and thus there is less wear to the cam mechanisms because of the distribution of the static and dynamic loads.
In a cost-effective embodiment, the gripper shaft acts as a torsion spring element in that the shaft is twisted to generate the torques of opposite directions. In accordance with a further feature, a torsion bar spring connecting the shaft ends and twisted to ensure positive contact can be provided inside a divided gripper shaft. The torques of opposing direction that act on the gripper shaft may furthermore be created by roller levers to which bending forces are applied.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a transfer drum for conveying a sheet in a machine of the graphic arts industry, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
Referring now to the figures of the drawing in detail and first, particularly, to FIGS. 1 and 2 thereof, there is shown a transfer drum of a sheet-fed printing press. A drum body 1 includes journals 2, 3 for supporting the drum body 1 in side walls of the printing press. The transfer drum is supported in the side walls so as to rotate about an axis 4. A cylindrical jacket surface 5 and a gap 6 are formed on the drum body 1. In the gap 6, a gripper shaft 7 is held in bearing blocks 8 so as to pivot about an axis 9. The axes 4, 9 of the drum body 1 and of the gripper shaft 7 are parallel. Roller levers 10, 11 including cam rollers 12, 13 are fixed to the ends of the gripper shaft 7. The cam rollers 12, 13 roll on contact surfaces of cams 14, 15, which are connected to the side walls and have identical cam laws. On each side, the roller levers 10, 11 are arranged on the cams in such a way that opposing torques act on the gripper shaft 7. As the drum body 1 rotates, the cam mechanisms including the cams 14, 15, rollers 12, 13, and levers 10, 11 cause an identical movement of the gripper shaft 7 to open and close the grippers 16 arranged on the gripper shaft 7. When the gripper shaft 7 moves, the twisting of the gripper shaft 7 remains constant. The roller levers 10, 11 are arranged on the gripper shaft 7 at an angle a relative to each other to ensure constant twisting and to cause the rollers 12, 13 to be pressed against the contact surfaces of the cams 14, 15 by a force. Thus neither static nor dynamic forces may cause the rollers 12, 13 to be disengaged from the contact surfaces.
FIG. 3 illustrates a detail of the gripper system. Each gripper 16 consists of a gripper finger 17 and a gripper pad 18. When the grippers 16 are in the closed state, a sheet 19 is held between the gripper fingers 17 and the gripper pads 18 due to the force of a spring 20. During transport, the sheet 19 rests on the jacket surface 5.
FIGS. 4 and 5 illustrate diagrams of the generation of the opposing torques M1, M2 on the gripper shaft 7. To assemble the cam mechanisms, the levers 10, 11 are fixed to the gripper shaft 7 at an angle relative to the gripper shaft 7. This angle is smaller than the angle a that both levers 10, 11 form when they are in the functional position. When the mechanism is assembled, lever 10 is pushed from the position illustrated in dashes into the position indicated in full lines, thus causing the gripper shaft 7 to be twisted. In accordance with the torsional stiffness, the torques M1, M2 of opposing direction are generated at the ends of the gripper shaft 7. The lengths I of the levers 10, 11 result in a contacting pressure F between the rollers 12, 13 and the cams 14, 15.
FIG. 6 is a diagram of a progress of the contact pressures F1, F2 of the rollers 12, 13 during one rotation of the drum body 1. When the grippers are open starting at an angle of rotation 1 of the drum body 1, the forces F1 and F2 are substantially the same due to a pre-load. When the grippers close at an angle 2 of the drum body 1, the force F1 of roller 12 increases whereas the load F2 of roller 13 decreases. The effect of the gripper forces is evenly distributed among the rollers 12, 13, so that the absolute change in the roller forces on both sides has the same amount.
FIG. 7 illustrates a variant in which the roller levers 10, 11 are located radially on the same side of a gripper shaft 7 as viewed in the direction of movement 21. To provide opposing torques at the ends of the gripper shaft 7, the roller 12 interacts with an inner cam 22 and the roller 13 interacts with an outer cam 23.
In the exemplary embodiments that have been described so far, the opposing torques M1, M2 were created by twisting the gripper shaft 7. In accordance with a variant shown in FIG. 8, a gripper shaft 7 is divided at the center. The torsion-proof and bend-proof sections 7.1, 7.2 are hollow, coaxial with respect to each other, and are freely rotatable with respect to each other about the axis 9 of the gripper shaft 7. The pretensioning of the cam mechanisms acting on the ends of the gripper shaft 7 is created by twisting a torsion bar spring 24 provided inside the sections 7.1., 7.2. The torsion bar spring 24 connects the outer ends of the sections 7.1, 7.2.