GB2197261A

GB2197261A - Offset printing machine dampening unit

Info

Publication number: GB2197261A
Application number: GB08723609A
Authority: GB
Inventors: Rudi Junghans
Original assignee: Heidelberger Druckmaschinen AG
Current assignee: Heidelberger Druckmaschinen AG
Priority date: 1986-10-09
Filing date: 1987-10-08
Publication date: 1988-05-18
Anticipated expiration: 2007-10-08
Also published as: GB8723609D0; GB2197261B; JPS63119033U; DE8626762U1; FR2604948A1

Abstract

An offset printing machine dampening unit, Fig. 1, comprises means whereby (in use of the unit) dampening fluid is applicable on to a printing plate from a supply container by way of dampening unit rollers and a dampening fluid applicator roller 13, the latter being disengageable from the printing plate cylinder, wherein the dampening fluid applicator roller has a roller casing 14 which is so mounted on a roller shaft 19 as to be rotatably and axially displaceable, said roller shaft being mounted in bearings 18 so as to be fixed against rotation, and the axial stroke of the roller casing being limited, on both sides, by two bushings 22,24. The arrangement is of value in assisting the prevention of "ghosting" on the printing plate and thus on the printed product, and the drive for the axial displacement of the applicator roller is obtained entirely by friction contact from the upstream dampening unit roller, the amount of displacement a being less than that of the upstream roller. <IMAGE>

Description

SPECIFICATION Offset printing machine dampening unit This invention relates to an offset printing machine dampening unit comprising means whereby (in use of the unit) dampening fluid is applicable on to a printing plate from a supply container by way of dampening unit rollers and a dampening fluid applicator roller, the latter being disengageable from the printing plate cylinder.

German Utility Model Registration No.8330123.2 shows an inking unit in which applicator rollers are driven by distributor rollers, and, as viewed in the direction of rotation of the printing plate cylinder, the last applicator roller performs an axial movement.

The purpose of the construction described is to reduce "ghosting" in an inking unit.

It is a disadvantage of this construction that it is not possible to use an applicator roller of the kind indicated with the same effect in a dampening unit, because the lower friction drive of the rollers carrying the dampening fluid is not capable of overcoming the force of certain pressure springs that are provided.

It is an object of the present invention to make possible-in a simple way an improvement in the known form of dampening unit such that, if the printing plate cylinder is equally dampened, the dampening fluid will prevent "ghosting" on the printing plate thus on the printed image.

According to the present invention, there is provided an offset printing machine dampening unit comprising means whereby (in use of the unit) dampening fluid is applicable on to a printing plate from a supply container by way of dampening unit rollers and a dampening fluid applicator roller, the latter being disengageable from the printing plate cylinder, wherein the dampening fluid applicator roller has a roller casing which is so mounted on a roller shaft as to be rotatably and axially displaceable, said roller shaft being mounted in bearings so as to be fixed against rotation, and the axial stroke of the roller casing being limited, on both sides, by two bushings.

In the unit of the present invention, the differential thicknesses of the dampening fluid film occurring on the dampening fluid applicator roller are shifted, through axial movement of said roller, so that there are no longer any sharp outlines and accordingly a "ghosting" effect on the finished printed image is hardly perceptible any more. Due to the gentle and free movement of the dampening fluid applicator roller, and due to the asynchronous characteristics of its movements with respect to the relevant distributor roller, the equalization of the dampening fluid film is improved, and this tends to prevent undue wear of the printing plate. In practice, a lateral movement of the dampening fluid applicator roller of the order of a few millimetres has been found to be quite adequate.In a preferred form of dampening fluid applicator roller, which avoids the use of pressure springs, the low friction drive of the rollers carrying the dampening fluid is sufficient to induce the desired lateral movement, by means of the relevant distributor roller.

An embodiment of the invention is shown in the accompanying diagrammatic drawings, in which: Figure 1 is a side view of an inking and dampening unit; and Figure 2 is a fragmentary longitudinal sectional view of the oscillating dampening fluid applicator roller of the unit of Fig. 1.

Fig. 1 shows an orthodox offset printing unit for printing a paper web 1 which passes between two blanket cylinders 2 and 3 cooperating with printing plate cylinders 4 and 5 respectively. The upper half of the printing unit seen in Fig. 1 has a dampening unit 6 which includes a dampening fluid supply container 28 for dampening the printing plate carried by the plate cylinder 4. An inking unit is shown at 7, and comprises an ink fountain 8, an ink fountain roller 9, a series of inking and distributor rollers, and ink applicator rollers 10, 11 and 1 2. For the purpose of impression throw-off, the ink applicator rollers 10, 11 and 12, and the dampening fluid applicator roller 13, are so mounted as to be disengageable from the plate cylinder 4.Also, the blanket cylinder 2 and the plate cylinder 4 are in this connection disengaged from each other, so that their outer cylindrical surfaces are no longer in contact.

Fig. 2 shows an oscillating dampening fluid applicator roller 14, comprising a roller casing 14 covered with an elastic coating 15, e.g. of rubber. By means of needle bearings 16 which have elongated inner bushings 17, the roller casing 14 is so mounted on a roller shaft 19 as to be rotatably and axially displaceable; the roller shaft 19, however, is mounted in b.ear- ings 18 so as to be fixed against rotation. In the illustrated embodiment the bearings 18 are provided inwardly of the side frames 20 and 21, and are so designed as to permit the disengagement of the ink applicator roller 12 during impression throw-off.

Within the roller casing 14, bushings 22 are fixed on the roller shaft 19 by means of clamping rings 23; these serve as stops for bushings 24 which are mounted within the roller casing 14 by means of ball bearings 25.

Between the bushings 22 and 24, there is a spacing a of about 4 mm when the dampening fluid applicator roller 13 is in its middle position; thus the dampening fluid applicator roller 13 can execute a lateral stroke of maximally ca. 8 mm.

Bushings 22 and 24 are provided on both sides of the dampening fluid applicator roller 13, these bushings being spaced from each other; the arrangement is such that the dampening fluid applicator roller 13 describes lateral movements wherein it follows, for the distance a, the distributor roller 27 with which it cooperates, and thereafter remains in position until the distributor roller 27 experiences a reversal of motion at its own limiting position. Then it is drawn back by the roller 27 to the other side, until the bushings 22 and 24 meet one another. Thus, an asynchronous lateral movement (with respect to the distributor roller 27) is executed by the dampening fluid applicator roller 13, which prevents "ghosting" without requiring an additional drive.

It will be understood that the invention has been described above purely by way of example, and that various modifications of detail can be made within the ambit of the invention.

Claims

1. An offset printing machine dampening unit comprising means whereby (in use of the unit) dampening fluid is applicable on to a printing plate from a supply container by way of dampening unit rollers and a dampening fluid applicator roller, the latter being disengageable from the printing plate cylinder, wherein the dampening fluid applicator roller has a roller casing which is so mounted on a roller shaft as to be rotatably and axially displaceable, said roller shaft being mounted in bearings so as to be fixed against rotation, and the axial stroke of the roller casing being limited, on both sides, by two bushings.

2. A unit according to claim 1, wherein the roller casing is mounted on the roller shaft by means of needle bearings which have elongated inner bushings, and each inner axialstroke-limiting bushing is rotatably mounted within the roller casing by means of a ball bearing.

3. A unit according to claim 1, substantially as described with reference to the accompanying drawings.