Sheet-transport drum and printing machine with a sheet-transport drum

A sheet-transport drum includes at least two sheet-supporting surfaces with suction openings. Each respective sheet-supporting surface is formed of first and second comb segments with segment teeth. Suction grooves are provided in the segment teeth and a trailing-edge suction groove is provided in the sheet-supporting surface of each respective second comb segment. A rotary valve is provided for a timed supply of suction air. A plurality of vacuum sources is provided and a trailing-edge suction groove is assigned to each respective vacuum source by the rotary valve. This ensures that the vacuum provided by a respective trailing-edge suction groove at the trailing edge of a respective sheet is reliably maintained even if a previous or following sheet is missing. A sheet-fed printing machine with the sheet-transport drum is also provided.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority, under 35 U.S.C. §119, of German Patent Application DE 10 2015 211 440.6, filed Jun. 22, 2015; the prior application is herewith incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to a sheet-transport drum in a sheet-processing machine. The drum includes at least two sheet-supporting surfaces with suction openings. Each respective sheet-supporting surface is formed of a first comb segment and a second comb segment. The comb segments have segment teeth with suction grooves formed therein. A trailing-edge suction groove is provided in the sheet-supporting surface of a respective second comb segment and a rotary valve provides a timed supply of suction air. The invention also relates to a sheet-fed printing machine, in particular a digital printing machine, including a sheet-transport drum with vacuum sources.

The use of digital printing machines to print shorts runs or customized printed images on sheets of paper, paperboard, and cardboard is known in the art. When inkjet heads are used to print on the sheets, a transport system moves a respective sheet underneath the inkjet heads at a minimum distance. Known transport systems are revolving transport belts, for instance embodied as suction belts, and rotating cylinders, also known as jetting cylinders, or revolving tables.

Machine concepts that use cylinders as described, for instance, in U.S. Patent Application Publication US 2009/0284561 A1, include a number of inkjet print heads disposed above a jetting cylinder to print on sheets that are moved past at a short distance from the print heads. A jetting cylinder may simultaneously hold a plurality of sheets by suction and transport them.

In order to guarantee a high printing quality and to avoid damage to the print heads, an important aspect is to ensure that a respective sheet rests securely on the jetting cylinder. If a sheet does not rest securely thereon and if dog ears stick out, for instance, they may damage the print heads. In order to prevent that, it is known to equip the printing machine with a detection device and to quickly stop the machine if there are any corners or edges that stick out. However, that means a considerable reduction of the throughput of the machine.

Sheet-transport drums including gripper systems for holding the sheets to be transported are known as devices for guiding sheets and are disclosed, for instance, in German Patent Application DE 42 21 046 A1, corresponding to U.S. Pat. No. 5,172,634, German Patent Application DE 101 02 226 A1, corresponding to U.S. Pat. No. 6,659,456, and European Patent Application EP 1 415 804 A1, corresponding to U.S. Pat. No. 7,150,456. The latter discloses a sheet-transport drum with sheet-supporting surfaces formed of two comb segments. In order to ensure that a sheet resting thereon is held securely, the comb segments have suction grooves that are supplied with suction air by using a rotary valve disposed inside the sheet-transport drum.

A disadvantage of such a sheet-transport drum is that the suction air breaks down if there is a sheet-supporting surface with no sheet resting thereon and the further sheets on the drum may no longer be held securely as a consequence. That is the case during the start-up and shutdown of the printing machine and when defective sheets are discharged before they reach the sheet-transport drum.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a sheet-transport drum and a sheet-fed printing machine with a sheet-transport drum, which overcome or at least reduce the hereinafore-mentioned disadvantages of the heretofore-known sheet-transport drums and printing machines of this general type and in which sheets are securely held on the sheet-transport drum even if previous or subsequent sheets are missing.

With the foregoing and other objects in view there is provided, in accordance with the invention, a sheet-transport drum in a machine for processing sheets, which comprises at least two sheet-supporting surfaces having suction openings wherein a respective sheet-supporting surface is formed of a first comb segment and a second comb segment with segment teeth embodied in a rake-like way and meshing with one another. Suction grooves are formed in the segment teeth of the first and second comb segments and a trailing-edge suction groove is provided in the sheet-supporting surface of a respective second comb segment, the trailing-edge suction groove being associated with a trailing edge of a sheet resting on the sheet-supporting surface. A rotary valve is provided to supply suction air to the suction grooves and to the trailing-edge suction groove. The sheet-transport drum advantageously includes a plurality of vacuum sources and the rotary valve exclusively assigns a single trailing-edge suction groove of a respective sheet supporting surface to a respective vacuum source. The suction grooves of the comb segments are likewise assigned to at least one vacuum source. This is achieved by a provision of corresponding channels or, to be more precise, supply lines and taps or air control openings in the rotary valve. The provision of a plurality of vacuum sources ensures that the vacuum that is present at the trailing edge of a respective sheet and is provided by a respective trailing-edge suction groove is reliably maintained even if a previous or subsequent sheet is missing.

In a particularly advantageous and thus preferred further development of the sheet-transport drum of the invention, the latter has a transfer region for receiving sheets from an upstream transfer cylinder. In the transfer region, a separate vacuum source is assigned to the suction grooves of the comb segments by the rotary valve. This ensures that in the transfer region, a respective sheet may reliably be “ironed” onto the sheet-transport drum, avoiding bulges in the sheet and preventing corners of the sheet from sticking out.

In an advantageous further development of the sheet-transport drum of the invention, a further vacuum source is provided to provide suction air to the suction grooves of the comb segments in the remaining area of the sheet-transport drums, i.e. outside the transfer region, by using the rotary valve.

It is furthermore deemed advantageous to operate the vacuum sources at different pressure levels, i.e. to use respective vacuum sources with an adjustable pressure, for instance to cause sheets to be held by a particularly strong suction effect in the transfer region and underneath print heads.

In an advantageous embodiment of the sheet-transport drum the comb segments have air control holes, also referred to as taps, that interact with the rotary valve to tap into the vacuum at the rotary valve. For this purpose, supply lines are located in the rotary valve. Thus a supply line to supply the trailing-edge suction groove, a further supply line to supply the suction grooves of the respective first comb segment, and a further supply line to supply the suction grooves of the respective second comb segment may be provided, i.e. introduced in the rotary valve.

In a particularly advantageous and thus preferred further development, the supply line of the trailing-edge suction groove is subdivided into separate sections in such a way that every section provides suction air to only one trailing-edge suction groove of a respective sheet-supporting surface. In other words, the length of the individual sections is selected in such a way that the suction air supply transitions to the next section before the supply of the subsequent trailing-edge suction groove of the next sheet-supporting surface is connected to the section. This definitely precludes any mutual influence between the different sheet-supporting surfaces. Furthermore, every section of the supply line of the trailing-edge suction groove may be connected to a separate vacuum source.

In an advantageous embodiment, the sheet-transport drum is embodied as a jetting cylinder for holding and transporting sheets to be printed in an inkjet process.

With the objects of the invention in view, there is concomitantly provided a sheet-fed printing machine comprising a sheet-transport drum as described above. The latter may be embodied as a digital printing machine, in particular one including inkjet heads disposed at a distance from and substantially above the sheet-transport drum.

Other features which are considered as characteristic for the invention are set forth in the appended claims. Inasmuch as this makes sense from a technical point of view, the invention described above and the advantageous further developments thereof described herein also form advantageous further developments of the invention in combination with one another.

Although the invention is illustrated and described herein as embodied in a sheet-transport drum and a printing machine with a sheet-transport drum, 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. Further advantages and embodiments of the invention that are advantageous in structural and functional terms will become apparent from the dependent claims and from the description of exemplary embodiments with reference to the appended figures.

DETAILED DESCRIPTION OF THE INVENTION

Referring now in detail to the figures of the drawings, which are not drawn to scale and in which mutually corresponding elements and components have the same reference numerals, and first, particularly, toFIG. 4thereof, there is seen a sheet-fed printing machine100embodied as a digital printing machine. A respective sheet1000coming from a feeder1is transported through a printing unit2to a delivery3in a direction of transport T. The transport of a respective sheet1000is mainly carried out by cylinders, namely transfer cylinders5and a sheet-transport drum10, embodied as a jetting cylinder. In the exemplary embodiment, the jetting cylinder10has three sheet-supporting areas21. Inkjet heads4are disposed above the jetting cylinder10. The inkjet heads4print on a sheet1000that is transported past by the jetting cylinder10at a short distance. Vacuum sources31,32,33,34,35for securely holding sheets1000by suction in the region of the sheet-supporting surface21and during transport and printing are assigned to the jetting cylinder10.

FIG. 2is a top view of the sheet-transport drum10andFIG. 3is a lateral sectional view of the sheet-transport drum10ofFIG. 2.FIG. 2shows that the sheet-transport drum10includes at least a first comb segment11and a second comb segment12. The second comb segment12is supported to rotate/pivot relative to the first comb segment11about a pivot axis13, which corresponds to the central axis of the sheet-transport drum10, to adapt the sheet-transport drum10to sheets1000of different format lengths to be transported. Each one of the comb segments11,12has teeth14that engage in interspaces between the teeth14of the respective other comb segment12,11.

The second comb segment12is adjusted towards the first comb segment11to accommodate a smaller sheet format by moving the teeth14deeper into the interspaces which causes the interspaces to narrow. In order to accommodate a larger sheet format, the second comb segment12is moved away from the first comb segment11, causing the interspaces to widen.

A gripper system15of the sheet-transport drum10has gripper pads16and gripper fingers17associated therewith on the front edge of the first comb segment11to clamp the sheet1000of printing material. A pneumatic gripper19has a trailing-edge suction groove18connected to a (non-illustrated) suction air source for holding the sheet1000of printing material by suction at its trailing edge is disposed on the second comb segment12. A number of suction nozzles disposed in a row may be used instead of the trailing-edge suction groove18. The mechanical grippers15at the leading edge of the sheets and the pneumatic grippers19at the trailing edge of the sheets are crucial for a secure transport of the sheets1000. The gripper systems15,19ensure that the sheets1000are held down at their corners and edges and that the sheets1000may be moved past underneath inkjet heads4at a defined, short distance. The first comb segment11and the second comb segment12are provided with suction grooves20that ensure that a sheet1000is held securely over its entire surface without any bulges.

FIG. 1Ais a more detailed sectional view of the suction air supply of the sheet-transport drum10. In the illustrated embodiment, the sheet-transport drum has four sheet-supporting surfaces21, each of which has a mechanical gripper15for gripping the leading edge, suction grooves20formed in the comb segments11,12for holding the sheet over its entire surface, and a pneumatic gripper19for holding the trailing edge of a sheet1000. A rotary valve22is disposed at the center/in the middle of the sheet-transport drum10to supply the suction grooves and trailing-edge suction grooves20,18with suction air. For this purpose, the trailing-edge suction grooves and suction grooves20,18are connected to the rotary valve22by suction lines27. The rotary valve22has a plurality of fixed supply lines23that interact with revolving taps or air control holes24,25,26and create a timed provision of suction air. The construction of the rotary valve22is shown in more detail inFIG. 1B.

The rotary valve22has three different supply lines23, which are disposed on different radii of the rotary valve22. The innermost supply line23serves to provide suction air to the suction grooves20of the first comb segments11. The middle supply line23serves to provide suction air to the suction grooves20of the second comb segments12. This ensures a reliable suction air supply even in the case of format changes. Finally, the outer supply line23serves to provide suction air to the pneumatic trailing-edge grippers19. Each respective supply line23is subdivided into three sections. The three sections of each respective supply line23are connected to different vacuum sources31,32,33,34,35.

In every segment of the sheet-transport drum10, each of which has a sheet-supporting surface21, i.e. the comb segments11,12, air control openings are provided, which will be referred to as taps below. Thus for every sheet-supporting surface21, there is a revolving tap24associated with the rotary valve22for the front suction grooves20of the first comb segment11, a revolving tap25associated with the rotary valve22for the rear suction grooves20in the second comb segment12, and a revolving tap26for the pneumatic trailing-edge grippers19of the second comb segment12.

Every section of a supply line23that interacts with the tap26to provide the pneumatic trailing-edge grippers19with suction air is connected to its own vacuum source. The first section is connected to a first vacuum source31, the second section to a second vacuum source32, and the third section to a third vacuum source33. This prevents any mutual influence in terms of the suction power and ensures that the pneumatic gripper19may attract and hold the trailing edge of the sheet by suction at any time. In addition, the rotary valve22has the following features:

The length of the individual sections of the fixed supply lines23in the rotary valve22is selected in such a way that the supply of suction air to the trailing-edge suction groove18transitions to the next section before the supply to a following sheet-supporting surface21is connected to the section of the fixed line23. Thus there is no mutual influence on the suction effect of the trailing-edge suction grooves18of the different sheet-supporting surfaces21, for instance because on one sheet-supporting surface21a sheet1000is missing.

The first sections of the supply line23, which interacts with the tap24for the front grooves and the tap25for the rear grooves, are connected to a separate vacuum source34. This means that in a transfer region of the sheet-transport drum10, a sheet1000may be “ironed” onto the sheet-supporting surface21over the entire surface of the sheet by an upstream transfer cylinder5. In further sections, a further vacuum source35is assigned to the central and outer supply lines23, which interact with the revolving taps24of the front grooves and the revolving taps25of the rear grooves. In order to reduce the mutual influence, respective throttling points36are provided between the sections of the supply line23and the vacuum source35in the supply lines.

The rotary valve22also has a region that is not provided with supply lines23. In this region there is an opening that causes aeration28at the trailing-edge suction groove and at the suction grooves18,20as soon as a sheet1000is transferred from the sheet-transport drum10to a downstream transfer cylinder5. The aeration28reduces the vacuum, aerates the trailing-edge suction groove and the suction grooves18,20, and thus facilitates a transfer of the sheet1000.