Device for removing soil particles from the plate cylinder of an offset printing machine
BACKGROUND OF THE INVENTION 
     1. Field Of The Invention 
     The invention relates to a device for removing soil particles from the plate cylinder of an offset printing machine, which device extends over the entire length of the plate cylinder and engages the plate cylinder in the region between the ink transfer to the rubber cylinder and the dampening. 
     2. Description Of The Prior Art 
     In offset printing, the plate cylinder becomes repeatedly soiled. The soils may be comprised of fibers of paper or cardboard from the material being printed on, balls or clumps of printing ink, or dust from the air. They can give rise to unprinted spots on the surface being printed. To eliminate this problem, either the printing machine must be stopped for hand removal of the soil particles or else a device must be applied against the rotating plate cylinder which device removes the soil particles. Stopping the press reduces the output and increases rejects which necessarily result whenever the press is stopped. On the other hand, engaging the running press with cleaning tools is hazardous to the operator, and is indeed prohibited. Further, because this (running-press) technique can only be carried out from the ink-application side, it results in an undesirable partial removal of ink. 
     Accordingly, there has been much research directed at means of removing soils from the plate cylinders of offset printing machines which means are free from these disadvantages. In this connection, solutions have become known of the types described, e.g., in Ger. OSs 25 22 743, 27 15 445, 28 50 448, 34 10 376, and also 36 20 156. The claimed matter of the first four cited German references involves devices in which a guide means is provided which is parallel to the rotational axis of the plate cylinder in the ink-free region of the plate cylinder, and which extends over the entire length of the plate cylinder. The guide means bears a driven cleaning implement. These known devices have not found acceptance in the art, however. The reasons for this are, first, that the space requirements for the guide means, cleaning implement, and drive are so great that the device is virtually impossible to accommodate in the very narrow region of the dampening system, and secondly, that the cleaning implement must be adjusted with each application, which requires high accuracy because of the sometimes only local point contact between the implement and the cylinder; and further, when soil particles are present at several locations they must be removed sequentially, which necessitates an extending cleaning time and therefore a high reject rate (amount of printed work which must be discarded). Ger. OS 3,620,156 discloses a device wherein a cleaning implement extends over the entire length of the plate cylinder, the implement having the form of an application cylinder of the dampening system. However, this device also does not produce the desired result. The means whereby the application cylinder is enabled to function as a cleaning implement are mechanically complex. Furthermore, the cleaning action is unreliable, because if the plate cylinder and application cylinder are not exactly parallel the soil particles will not be removed over the entire length of the cylinder. In addition, the force of the application cylinder against the plate cylinder is relatively low, whereby the result may be that the soil particles are only mashed down instead of being removed. 
     BRIEF SUMMARY OF THE INVENTION 
     The object of the invention is to provide a cleaning device which does not have these drawbacks of known devices, but enables the plate cylinder of a running offset printing machine to be cleaned in a manner which is safe for the operator, is rapid, and is fully efficacious, while generating a minimal number of rejects. The starting point of the invention which solves this problem is a known device of the type described initially supra. The problem is solved in the inventive device in that a plurality of pneumatic cylinders are disposed distributed over the entire length of the plate cylinder, having piston plungers which are non-rotatable and which bear corresponding elastic wiping implements on their free ends. Neighboring wiping implements have a reversed orientation with respect to each other and laterally overlap, and the attainable excursion length of each piston plunger is slightly greater than the distance between the forward edge of the wiping implement in the retracted position and the surface of the plate cylinder. 
     With this arrangement, the wiping implements form a closed front which covers the entire length of the plate cylinder, so that when the offset printing machine is in operation the front can be brought against the surface of the plate cylinder segmentally. Accordingly, if only one soil particle is present, it is sufficient to advance only a single segment, i.e., a single wiping implement, whereas if several soil particles are present, distributed over the length of the plate cylinder, they can be removed in one stroke by advancing several segments, i.e., several wiping implements. The alternating reversed orientation and overlapping sides of the wiping implements prevent development of a dead zone between neighboring wiping implements. The length chosen for the available excursion ensures firm pressing of the advanced wiping implements against the plate cylinder under all conditions, as well as compensation for any wear which may occur to the wiping implements. The elasticity of the material of the wiping implements, in combination with the yielding nature of the pneumatic drives, ensures that the surface of the plate cylinder will not suffer damage. Finally, compact pneumatic cylinders of a type currently commercially available may be employed to keep the space requirements for the device low, whereby the device may readily be mounted on the side of the dampening system. 
     Accordingly to an advantageous refinement of the invention, the pneumatic cylinders (singly, or jointly in unison) are controllable in lag-free fashion, i.e. with the least possible time-lag between the actuation and operation of the pneumatic cylinders, from the control panel of the printing machine, and are affixed to a strip which extends over the entire length of the plate cylinder and parallel to the longitudinal axis of the cylinder. The piston plungers of the pneumatic cylinders all have the same excursion length. The cleaning implement is actuated from the same position from which the ink zone screws are controlled to adjust the ink layer thickness. The affixing of all the pneumatic cylinders to a single strip enables substantially complete preassembly of the device, and simplifies installation and removal. 
     It has proven particularly advantageous if the wiping implements are each comprised of a rear base part and a front contact blade, both of which have a generally rectangular shape, with the base part being substantially thicker and longer than the contact blade, and if the contact blade has lateral extensions on both sides of the base part. The thickness and length of the base part should be about three times the respective dimensions of the contact blade. This produces conditions whereby, according to another feature of the invention, a mounting opening for inserting the piston plunger may be provided which opening extends centrally into the base part from the rear side. The comparatively weak structure of the contact blade, which extends preferably about 5 mm on each side beyond the base part, facilitates elastic deformation of the blade when the wiping implement is pressed against the plate cylinder. 
     According to a particularly advantageous feature of the invention, the width, i.e., along the cylinder axis, of each base part is equal to the width of the corresponding ink zone appearing on the control panel of the offset printing machine. This correspondence greatly facilitates the operation of the cleaning implement, because it eliminates the tedious process of determining which wiping implement should be actuated to remove a given soil particle, and enables actuation &#34;at a glance&#34;. 
     In order to prevent a situation where soil particles which adhere to the wiping implements after they have been removed from the plate cylinder are then redeposited when the wiping implements are reapplied to the plate cylinder, it is recommended to provide a doctor blade which can be applied against the bottom of each wiping implement on its retraction stroke, thereby scaping off adhering soils. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A preferred exemplary embodiment of the inventive cleaning device will now be described in detail with reference to the drawings wherein: 
     FIG. 1 is a top plan view showing an assembly of the wiping implements and actuators in accordance with the invention, the plate cylinder being shown in cross section and the control panel shown schematically; 
     FIG. 2 is a front elevational view of the wiping implements only of FIG. 1; and 
     FIG. 3 is a cross-sectional view taken along line III--III of FIG. 1. 
    
    
     DETAILED DESCRIPTION 
     Five pneumatic cylinders 1a-1e are shown in FIG. 1, having corresponding housings 2a-2e with dimensions of 48×12×5 mm and piston plungers 3, of which only legs 3a and 3c are visible which are part of the forked construction of the plungers, with the free ends of the respective two legs of the fork being connected by a crosspiece 4 (of which only 4a and 4c are indicated in FIG. 1). The compressed air lines 5a-5e to each pneumatic cylinder are schematically as connected to a control panel 16 also shown schematically and having mounted thereon pneumatic controls 15a-15e which are manually operated for connecting each air line to a compressed air source (not shown) for selectively operating the pneumatic cylinders 1a-1e. 
     The pneumatic cylinders 1a-1e are attached by screws to a strip 6 which passes through the entire device extending the length of the plate cylinder. For purposes of this attachment, the cylinders 1a-1e are supplied with mounting lugs 7a-7e having holes 8a-8e, and also have bores 9a-9e through the housings. The mounting screws passing through bores 9a-9e extend between the two forks of the piston plungers 3. 
     Wiping implements 10a-10e comprised of elastic material are pushed over the free ends of the piston plungers 3 and crosspiece 4. These implements are comprised of respective rear base parts 11a-11e and front contact blades 12a-12e each having a forward edge for contacting the outer surface of the plate cylinder. The base parts 11a-11e are substantially thicker and longer than the contact blades 12a-12e, and each base part has a mounting opening 13 for receiving the piston plunger 3 and crosspiece 4. As seen particularly in FIG. 3, the base part 11a-11e undergoes a curved transition into the contact blade 12a-12e, and each such blade has lateral extensions 12a1, 12a2; . . . 12e2 on each of its two sides. 
     As seen from FIGS. 1 and 2, the wiping implements 10a-10e as a set oriented with respect to each other such that the contact blades 12a, 12c, and 12e of the first, third, and fifth wiping implements 10a, 10c, and 10e are on the underside, and the contact blades 12b and 12d of the second and fourth wiping implements 10b and 10d are on the top side, whereby adjacent lateral extensions (e.g., 12a2 and 12b1) overlap. In this way a closed front of the wiping implements 10a-10e is formed which covers the entire length of the plate cylinder 14. In FIG. 1, wiping implements 10b, 10d, and 10e are shown in retracted position, and wiping implement 10a is shown in advanced position (pressed against the surface of the plate cylinder). 
     Wiping implement 10c is shown in an even farther advanced position than 10a, which is slightly beyond the normally attainable position of wiping implement 10c. It is shown merely to indicate the maximum excursion length, amounting to about 10 mm with pneumatic cylinders 1a-1e of the dimensions stated supra. Because the maximum excursion length is slightly greater than the distance between the forward edges of the wiping implements 10a-10e and the parallel surface of the plate cylinder 14, when the pneumatic cylinders 1a-1e are actuated the wiping implements 10a-10e will always be firmly pressed against the surface of the plate cylinder 14, and any wear which has occurred on the contact blades 12a-12e will be compensated for. 
     As mentioned above, a doctor blade 18 can be provided for applying against the bottom of each wiping implement during its retraction stroke for scraping off soils adhering thereon. An example of such a doctor blade is shown schematically in FIG. 3.