The main purpose of calendering is to bring the surface properties and/or thickness of paper to a desired level. In the calendering treatment a continuous paper web is guided via nips formed by rolls. It is a known process to use a roll equipped with a soft roll coating, for example a polymer coated roll in the calendering. In calenders, polymer-coated rolls form a soft nip with their counter roll, said nip being longer than the nip between hard rolls, and the local surface pressure exerted on the paper web is smaller therein than in the corresponding"hard"nip.

Calenders can comprise superimposed rolls, between which calendering nips are formed. The nip can also be formed between a polymer-coated roll and a hard counter roll of the same in a single calender roll pair, and the calender can contain two such pairs successively to attain two-sided treatment. The roll positioned against the polymer-coated roll is typically a heated so-called thermoroll by means of which temperature can be used as a variable in the calendering of paper, but which causes some problems.

The surface of the polymer-coated roll is subjected to a thermal stress dependent on the temperature of the heated counter roll, i. e. the thermo roll, in those edge areas of the roll in which the paper web to be calendered that is passed through the nip is not located between the counter roll and the polymer coated roll.

Known solutions for preventing the damaging of the polymer coating in the nip between the polymer-coated roll and the thermo roll include for example the oscillation of paper from one edge to another, wherein the average temperature in the ends of the roll is reduced because the paper protects the ends of the roll. This arrangement is known in off- line calenders.

Another known solution is to run the paper in overwide form and cut the edges off after the calender. Such a solution is known for example from the patent publication FI 83249 and in the corresponding publication GB 2218434.

The publication US 5,266,167 discloses a cooling system for a polymer-coated roll in which the ends of the roll are cooled down with a water film. The water film is produced by means of a brush, sponge or felt cloth which is connected to the water source and can be moved in the axial direction of the roll.

The publication EP 792966 and the corresponding US patent 5,816,146 disclose a cooling system for a polymer-coated roll in which the ends of the roll are protected with an insulating band arrangement.

Advantageously the insulating band is a looped band which travels over at least one reversing roll and which is tensioned with a tensioning device.

It is a problem of known solutions that despite of the act of monitoring the temperature of the edge areas of the polymer coating, and the different cooling and insulating systems, damages occur in the coating.

On the other hand, when the paper is run overwide through the nip, the operating efficiency is increased and losses are caused by the edge strip to be cut off.

Especially in calenders in which high temperatures of the counter roll and large nip loads are used, damages in the roll coating and in the edge areas of the same pose a problem. Some of the damages are caused by the fact that the roll has become dirty or the coating of the roll is worn in the area of the web, wherein contact to the counter roll cannot be avoided. The thermal expansion of the polymer coating in the edge area with a higher temperature may also contribute to the development of the direct contact.

To prevent the damages, it is known to service the roll at intervals. For example by means of a grinding device the dirt is ground off or a so-

called micro-bevel is produced on the edge of the coating. At the moment the micro-bevel is made manually by means of a grinding pad, when the exact location of the web with respect to the coating is known. Problems are caused by the fact that manual grinding is more or less imprecise, and it cannot be conducted safely during the run.

The calender must then be stopped for maintenance operations.

Although it is possible to avoid damages in the edge areas of the polymer coating by means of grinding performed at right times, it is problematic that grinding causes an interruption of use in the calender.

The international publication WO 99/36616 discloses a method developed for avoiding faults in the paper web, in which the calender roll is ground during the run by using a traversing grinding device whose function is automatic by means of measuring devices controlling the shape of the roll and recognizing the faults on the surface of the same.

One purpose of the invention is to eliminate the drawbacks in the calendering process outside the area covered by the web in particular, and to introduce a calender by means of which it is possible to avoid the damaging of the soft roll coating of the roll, forming a nip with the heated counter roll in the edge areas. To attain this purpose the calender according to the invention is primarily characterized in that the grinding device containing a grinding tool and an actuator which is arranged to transfer the grinding tool against the roll coating, is placed in the vicinity of the roll coating, in the area remaining outside the edge of the web to be calendered. As a result of the arrangement it is possible to conduct measures preventing the damaging for example during the operation of the calender. Furthermore, when the grinding device is fixed precisely to the supporting structure, it is possible to conduct grinding with great accuracy for example to produce a micro- bevel of an accurate angle in the edge of the roll coating.

The function of the grinding device can be automated in such a manner that the actuator that transfers the grinding tool against the roll coating obtains its commands on the basis of the information received from the

temperature measurement. Thus, into the unit controlling the function of the grinding device it is possible to determine for example the threshold value of the surface temperature in which the grinding device starts to conduct grinding. The temperature sensor is placed in such a manner that it measures the temperature of the coating at that point in particular in which the production of damages should be prevented, especially in the same edge zone of the coating which will be subjected to grinding.

In the following, the invention will be described in more detail with reference to the appended drawings, in which Fig. 1 illustrates the structure of a roll equipped with a soft roll coating and the location of the counter roll of the same and the paper web, Fig. 2 shows a side view of the roll and the grinding device, and Fig. 3 shows the grinding device and the roll on a larger scale at the location of the edge of the coating.

Fig. 1 shows two rolls of the calender that are arranged rotatable, the lower of the rolls being a roll P equipped with a soft roll coating, such as polymer coating, that forms a calendering nip N with the heated counter roll, a so-called thermo roll T. The frame of the calender is not shown and only the lower part of the shell of the thermo roll is shown.

The soft-faced roll P can be for example a roll made by the applicant equipped with a"Dura"coating, the surface roughness Ra of which is of the order of 0.1 to 0.4 Rm, and the hardness of which varies in the range of 85° to 93° Sh. D, but the invention is not restricted solely to these rolls. The heated counter roll T can be a metal roll with a hard surface, for example a chill cast roll. The paper web W to be calendered travels through the nip N, wherein it is subjected to the effect of temperature and pressure in a manner known as such. The arrangement according to Fig. 1 can be in a calender, in which the nip is formed between the rolls in a calender roll pair, wherein two such pairs can be located successively, or the arrangement can also be

located in a stack of calender rolls comprising several rolls, wherein the described roll equipped with a soft roll coating can be for example the intermediate roll in the stack of calender rolls.

The soft roll coating P1 is placed on top of a straight cylindrical shell P2 that functions as a core for the roll, in such a manner that it covers the shell at least in the area of its central part, in which the web to be calendered travels. As can be seen in the figure, the shell of the roll can be exposed in both ends of the roll, i. e. the coating does not extend all the way to the outer edge of the shell. Furthermore, both edges of the coating are equipped with a bevel V within a narrow zone, in which bevel the coating gradually becomes thinner towards the end of the roll. Between the bevels V there remains a straight zone of the roll coating P1, which is intended to be in contact with the web W to be calendered that is passed through the nip. In this context, the term straight refers to the shape of the outer surface resulting from the evenness of the coating in the axial direction of the roll, without taking into account possible deflections of the roll.

The area covered by the web W in the nip N is in Fig. 1 the area remaining between the dotted lines. As can be seen in the figure, the web W does not cover the coating P1 entirely in the nip in the section between the bevels V of the same, and neither does it cover entirely the straight shell surface of the counter roll T on the opposite side.

Thus, there is an area outside the edges of the web W, in which the hot shell surface of the counter roll T is located directly opposite the edge area of the coating P1 remaining between the bevel V and the outer edge of the web W, and this causes the risk of damages described hereinabove in the introductory part. To avoid damaging, a grinding device G is positioned by said edge area, by means of which it is possible to perform the grinding, if necessary. The grinding device G is placed within a suitable distance from the nip N itself in the direction of the periphery of the roll.

Fig. 2 shows in more detail the structure of the grinding device G. The grinding device is placed in the transverse beam B of the calender, and it contains a grinding tool 1, for example a diamond grinding pad which

is fixed to a sledge 2 that is arranged to move in the radial direction with respect to the roll P along a guide located in the body 3 of the grinding device by means of an actuator 4 effecting a linear movement, advantageously by means of a pressurized medium operated actuator, such as a pneumatic cylinder. The actuator 4 can also operate electrically. The opposite ends of the actuator 4 are fixed to the backside of the sledge 2 and to a part protruding from the body 3. The body 3 of the grinding device itself, which is always within a fixed distance from the roll coating P1 in the radial direction is fixed to the transverse beam B or to a corresponding supporting structure connected to the frame of the calender. The grinding device G can be arranged to move in the transverse direction for example for that purpose that it can always be arranged in a standby position opposite to the edge area of the coating of the calender roll remaining outside the web, in which standby position it is during the running of the calender, when grinding is not necessary.

Fig. 2 shows the grinding device G in the working position in which the actuator 4 has with a short linear movement in the radial direction of the roll P transferred the grinding tool 1 that has been away from the contact with the coating P1 in the standby position, in contact with the roll coating P1. The curvature of the grinding surface of the grinding tool in the cross-section of the roll P corresponds to the curvature of the outer surface of the coating P1, wherein the grinding tool 1 is in contact with the coating P1 within a fixed length along the periphery of the same.

Fig. 3 shows the location and structure of the grinding tool G in a view perpendicular to the view of Fig. 2. The figure shows how the surface of the grinding tool 1 located opposite to the roll coating P1 is also slightly diagonal with respect to the axial line of the outer surface of the coating P1 in such a manner that it is possible to produce a micro- bevel in the outer edge of the straight portion of the coating, i. e. a part tapering conically towards the end of the roll P. In Fig. 3, the angle a corresponding to the micro-bevel, i. e. the angle of inclination of the grinding surface of the grinding tool 1 with respect to the coating is

considerably exaggerated. In practice, the angle is under 0.5°, for example approximately 0.2°.

The grinding tool G can be operated manually by means of a control panel, if the user detects signs of thermal damage risks, for example dirt in the edge of the coating. It is, however, possible that the device functions automatically when a fixed threshold value correlating with the risk of damages is attained. Fig. 1 shows a temperature sensor S placed in the same zone with the grinding device G but within a suitable angular distance from the same in the peripheral direction, said temperature sensor S communicating with a comparator unit via a data transmission line D, in which comparator unit a fixed temperature value (threshold value) is set beforehand. This thresholdvalue must not be reached by the edge zone of the roll coating P1 outside the web W.

The sensor S measures the temperature of the edge area of the roll coating P1 remaining outside the edge of the web W. If the temperature measured by the sensor S reaches this value, the comparator unit C transmits a command via the control line L to the actuator 4 of the grinding device G to transfer the grinding tool 1 against the coating P1. The grinding may be under way for a predetermined period of time at the same time when the roll P rotates and performs calendering of the paper web. Thereafter it is again possible to monitor the temperature. If the temperature does not fall underneath the threshold value after a set time, the procedure is repeated. Alternatively, it is possible to continue grinding for such a long period of time that the temperature falls below the threshold value.

The actuator 4 is advantageously a pressurized medium operated actuator, wherein the pressure of the pressurized medium therein, for example in the case of pneumatic cylinder, the pressure of air, at the same time determines the grinding pressure. The actuator can also operate electrically, wherein the grinding pressure can be made dependent on an electric variable. The measurement of temperature by means of the sensor S can be implemented with a known measurement method which can be used for measuring surface temperatures.

Fig. 2 also shows liquid nozzles 6 located within a suitable angular distance from the grinding device G, said liquid nozzles being connected to a source of a cooling agent, wherein at the same time with the grinding it is possible to spray cooling agent, for example water onto the coating. Thereby it is possible to rapidly cool down the roll coating P1 to minimize the risk of damages. Thus, the grinding pad dries the roll and prevents the cooling agent from splashing. In order to allow the cooling agent to act for a sufficiently long period of time, the nozzles are positioned directly after the grinding device G in the direction of rotation, for example in an angular distance of 90° therefrom. The nozzles 6 can be coupled to operate simultaneously with the grinding device G under the control of the comparator unit C.

The invention can be applied in all calenders containing a combination of the above-described roll equipped with a soft roll coating and heated counter roll. Furthermore, the calender in which the grinding device is used may be an on-line calender located in the production line of paper or an off-line calender located in a separate finishing apparatus. The grinding device G and one or more cooling agent nozzles 6 possibly located in the same edge zone may be positioned in both edges of the coating P1, although Fig. 1 shows a grinding device G located only on one edge.