GB2221857A - Cylinder coating device - Google Patents

Abstract

A coating device comprises a coating cylinder 1 rotating in a sump (4) of coating mass and defining a coating gap (6) of finite width with a counter cylinder (2), and a doctor blade (10) downstream from the coating gap. The coating or counter cylinder is designed as a sag-compensating cylinder, comprising a stationary yoke 12, a rotatable cylindrical cover 13 which is placed concentrically around the yoke, and hydraulic supporting devices 14 located between the two. <IMAGE>

Description

COATING DEVICE.

The invention relates to a coating device in accordance with the pre-characterising clause of the principal claim. Such coating devices are known from "Wochenblatt fur Paplerfabrikatlon (Weekly magazine for paper making) 1979, pp. 137 ff Figure 4.

In order to ensure uniform appllcation of the coating paint, great accuracy is required with regard to both the coating and the removal of excess paint in the doctor blade station. The requirements with regard to accuracy are of the order of magnitude of hundredths of a millimetre whereas the disturbing influences, triggered by various operating conditions such as, in particular, variations in temperature, are of an order of magnitude of mlllimetres.

There is a major challenge in trying to keep the gap width between the coating cylinder and the counter cylinder constant. For a given application, this gap must have an exact dimension which must be maintained as precisely as possible over the entire length of the gap, i.e. the width of the web. If this is not done, there is no way of correcting It completely by means of the doctor blade station.

The most varied steps have been taken to achieve this goal, i.e. to maintain a constant gap size over the entire width of the web. One of these steps was to crown the coating cylinder. This compensates for the sagging of the cylinder; the longer the cylinder, the more important this becomes. Attempts were also made to solve the problem by using a design of particularly high flexural strength.

However, none of these methods was completely satisfactory. It was found again and again that it is not possible to maintain a constant gap size.

The purpose of the invention is to design a coating device according to the generic concept in such a manner that the gap between the coating cylinder and the counter cylinder Is always held at a constant size.

This purpose is achieved by the characterising features I n that the coating cylinder or the counter cylinder is designed as a sag-compensating cylinder i.e.

(that it comprises) a stationary yoke, a cylinder cover which is placed concentrically to the yoke and is rotatably supported, as well as a hydraulic supporting device located between these two, e.g. a pressure chamber or a row of support units.

The inventors have found the following: The coating cylinder Is wading in a sump of coating mass.

This sump generates a lifting force that acts on the coating cylinder. The lifting force depends on a number of factors, Including, in particular, the specific weight of the coating mass which, in turn, depends on the consistency of this mass, and the varying speed of the coating cylinder.

As regards presses, it Is common to replace a crowned cylinder by a so-called floating cylinder.

In presses, this has a specific purpose: Crowning of the press cylInders is to keep the linear pressure in the pressure gap as constant as possible over the entire web width (= gap length). This is also achieved when a certain pressure and a certain amount of crown adapted to that pressure exists. If the pressure changes, however, a different shape of crown is required to keep the linear pressure constant over the entire web width. In this respect floating cylinders are superlor to crowned cylinders since they make it possible to take care of pressures of widely different values.

In presses, the two press cylinders in question touch each other in the area of the pressing gap unless a paper web or similar object is running through the pressing gap.

In coating devices, on the other hand, no contact is made between the cylindrical surface of the coating cylinder and the cylindrical surface of the counter cylinder; instead, a gap of finite width exists between these two cylindrical surfaces.

At first It was Inexplicable why the gap width, despite the crown-shape (of the cylinders), deviated again and again from the specified value. Only as a result of the findings mentioned above -- effect of the lifting force -- was Is possible to solve the problem.

Without these findings, there would have been no reason to change from the usual crowned coating cylinder to the expensive sag-compensating cylinder which has a complicated design.

The Invention Is explained in more detail with the aid of the drawing: Figure 1 shows a side view of a coating device with coating cylinder, counter cylinder and doctor-blade device; and Figure 2 shows an axial section and, on an enlarged scale, one of the end sections of coating cylinder 1 from Figure 1.

The coating device shown in Figure 1 comprlses, as main elements, a coating cylinder 1 a counter cylinder 2 and a doctor blade device 3 The coating cylinder 1 "wades" in a sump 4 of coating paint. The sump has a feed, not shown here, and an overflow 5.

Coating cylinder 1 and counter cylinder 2 together form a coating gap 6 so that in the idle, i.e.

the non-operating conditlon, a distinct distance exists between the cylindrical surfaces of cylinders 1 and 2. A paper web 7 loops around counter cylinder 2 which turns in the direction of the arrows, with the two cylinders 1 and 2 also turning In the direction of the (respective) arrows.

When the cylinders rotate, coating cylinder 1 carries coating paint from sump 4 to the coating gap 6, and in doing so It transfers coating paint to paper web 7. A small amount adheres to the cylindrical surface of coating cylinder 1 and Is returned to sump 4 as reflux 8.

Paper web 7, carrying coating 9, then arrives at the doctor blade station 3. The scraper 10 in the doctor blade station evens out coating 9. Paper web 7 then separates from counter cylinder 2, together with the evened-out coating on the latter, while the excess 11 of coating paint Is removed by scraper 10.

Coating cylinder 1 shown in Figure 1 is a sagcompensating cylinder. As main components it has a stationary yoke 12, which is supported in such a manner that It cannot rotate, a cylindrical cover 13 which is supported in such a way that It can rotate, and a large number of support units 14 distributed over the length of the cylinder (= length of paper web 7). At its ends, the yoke 12 is supported in spherically shaped bearings 15 In order to take care of flexing of the yoke. The cylindrical cover 13 Is supported In bearings 16 at Its ends.

The support units 14 are uniformly distributed over the entire length of the cylinder, in the area of the coating gap 6 of the cylinder 1. They comprise the pistons 17, located In cylindrical recesses 18 of yoke 12. The pistons 17 are in contact with the Internal cylindrical surface of the cylindrical cover 13. When the cylindrical cover 13 rotates, the faces of the pistons 17 slide on the Internal cylindrical surface of the cylindrical cover 13. Each cylindrical recess 18 Is equipped with a connection 19 for a pressure medium.

Coating cylinder 1 wades during the operation in sump 4 of coating paint (see Figure 1). In this arrangement, the weight of coating cylinder 1 exerts a downward force, and the lifting force of the sump exerts an upward force on that cylinder. The resulting elastic I ine depends on many factors whose Individual effects cannot be computed accurately In advance. In addition to the weight of the cyllnder, these factors Inciude, among other things, the cylinder length, the resistance moment of the cylinder, the speed of the cylinder and the specific weight of the coating paint In the sump.

By applying the appropriate pressure to the cylindrical recesses 18 through the pressure connections 19, the elastic line of the cylindrical cover 13 can be precisely adjusted. This makes it possible also to control the width of the coating gap 6. The result shows in the thickness profile of the coating on the paper web after It has left counter cylinder 2 and has passed under the doctor blade.

Thus, this thickness profile, I.e. the coating thickness measured across the width of paper web 7, can be measured and be used, for example, as control parameter for the Individual pressure cyllnders 17. On the other hand, one could also consider using the gap width 6 directly as a measured variable.

It may also be useful to provide for cylinder 1 not only a single row of support units 14 but also a second row (not shown here) which is shifted by 1800 against the first row. This results In greater stability and quieter running of coating cylinder 1. In addition, this makes It easier to Install and remove the cylindrical cover 13 of coating cylinder 1 and makes It possible to correctly compensate the opposing forces of the cylinder weight and the lifting force. Of course, the invent ion also makes it possible to compensate the sagging of the counter cylinder.

Instead of the coating cylinder It is also possible to design the counter cylinder 2 as a sagcompensating cylinder. However, because of its smaller diameter, the coating cylinder is better suited for this.

Claims (5)

1. Coating device comprising a coating cylinder arranged to rotate in a sump of coating mass, a counter cylinder forming a coating gap of finite width with the coating cylinder, and a doctor blade station downstream from the coating gap, characterised in that the coating cylinder or the counter cylinder comprises a stationary yoke, a rotatably supported cylindrical cover and a hydraulic supporting device located between the yoke and cover.

2. Coating device according to Claim 1, characterised In that the hydraulic supporting device is placed in the area of the coating gap.

3. Coating device according to Clalm 1 or 2, characterised in that at least two supporting units are provided, which -- viewed in a cross sect ion perpendicular to the axis -- are, on the periphery of the yoke, shifted In relation to each other by a given angle.

4. Coating device according to Claim 3, characterised in that one or several supporting units are placed in the area of the coating gap, and one additional or several supporting units are located in positions shifted by 1800 in relation to the first-named supporting units.

5. A coating device substantially as herein described with reference to, and as shown In the accompanying drawings.