Typically, calenders comprise superimposed rolls, between which a nip is formed. The nip can also be formed between a so-called shoe roll and its counter roll or between a belt loop travelling through the nip and its counter roll. The rolls may be unheated or heated, and they can have a hard or soft surface. Between the polymer-coated belt and its counter surface a nip is formed which is longer than the nip formed by the hard rolls, and in which the treatment time of the web becomes longer than that in the nip formed by the hard counter surfaces, but the pressure exerted on the web per surface area unit is small. By means of the shape of the surface, such as a shoe, behind the polymer sur- face, and/or the elasticity of the polymer material, such as a belt, it is possible to affect the length of the nip. The web travels via the nip, and by means of this treatment for example the variations in the thickness of the web are reduced, the surface becomes smoother and the web becomes thinner.

A calender in which the nip is formed between a heated roll and a belt is called a long nip calender. In a long-nip calender the nip pressure can be adjusted in the transverse direction of the machine, wherein it is possible to profile for example the thickness of paper or paperboard.

The long-nip calender can be formed of a belt calender in which a belt is by means of auxiliary rolls guided around a second nipping roll that functions as a counter roll for thermo rolls. Thus, the long nip is formed between the thermo roll and the belt, which is loaded with a second nipping roll. The belt calender can be composed of a normal thermo roll, a belt loop and a counter roll of a soft calender, which counter roll can be either a hard roll or a soft roll. The belt rotates via the counter

roll and guiding/tension rolls. The simple structure of the belt loop also enables the modernizing of old machine calenders and soft calenders into belt calenders. Without a belt the calender can be used as a con- ventional machine calender or a soft calender depending on the counter roll.

The most common long-nip calender is a shoe calender in which the belt is arranged to travel around a stationary supporting structure, and in which the belt is loaded against the thermo roll by means of a loading shoe that rests on the supporting structure inside the belt loop. In the shoe calender, the long nip is formed between the thermo roll and the belt, which is loaded by the shoe. Thus, the loading shoe of the shoe calender defines the length of the nip. One example of the shoe calen- der is disclosed in the publication WO 9928551, and the shoe used in the press section is disclosed in the publication US 4705602.

In this application the concept of a polymer coating refers to a counter surface in which the surface that forms the nip is a polymer-coated belt.

Such solutions are, for example, the polymer-coated belt of the shoe calender or a belt arrangement in which the polymer-coated belt travels around the rolls and via the nip.

The polymer-coated belt is resilient and it is often pressed in the nip in such a manner that spreading of the belt takes place in the transverse direction of the web, wherein the section of the web that is exposed from underneath the web is easily damaged under the effect of heat and/or wear. The polymer-coated belt forms a rotating belt loop in the calender, and the damaging is facilitated by the fact that there is not enough time for the belt to cool down before it enters the nip again. A known solution for preventing the damaging of a polymer-coated belt in the nip between a polymer-coated belt and a thermo roll is for example a solution in which the width of the belt is dimensioned according to the smallest width of the paper web. It is a problem of the solution that when webs wider than the minimum width are run in the calender, part of the production is lost.

The method according to the invention is characterized in that the polymer-coated belt is moved in the transverse direction of the web to be treated. The calender according to the invention is characterized in that the calender comprises means which are arranged to move the polymer-coated belt in the transverse direction of the web to be treated.

It is an aim of the invention to reduce the problems detected in connec- tion with known solutions. By means of the method according to the in- vention it is possible to prevent the damaging of a polymer-coated belt as a result of an increase in the temperature and/or excessive local stress in an efficient and simple manner.

The polymer-coated belt is arranged to be moved in such a manner that it moves in the transverse direction of the web to be treated. If neces- sary, the polymer-coated belt can be moved when the aim is to align the edge of the web with the edge of the counter surface. Thus, the polymer-coated belt can remain stationary in the transverse direction of the web to be treated after the adjustment when the rotating movement of the polymer-coated belt which is parallel to the travel direction of the web continues or its alignment with the edge can be monitored by means of a suitable monitoring device and adjusted, if necessary.

It is also possible that the measurement of temperature is arranged in the edges of the polymer-coated belt and the motion of the polymer- coated belt is arranged to start when the temperature exceeds a certain set value. To attain a movement in the transverse direction of the poly- mer-coated belt, the calender is provided with members suitable for this purpose. The members can, for example, be suitable pushers which push the roll frame in the axial direction at a speed suitable for the pur- pose. The pusher can be for example a hydraulic cylinder or a pneu- matic cylinder, or a screw arrangement that initiates movement in the transverse direction of the web to be treated.

In the calender according to the invention the polymer-coated belt can be moved in the axial direction. On the other side of the belt, for exam- ple on the so-called driving side, the belt is aligned very accurately with the edge of the paper web. On the edge where the edge of the paper

and the polymer-coated belt are in alignment with each other, over- heating of the polymer coating can be avoided, because the paper web protects the polymer-coated belt. The problem still remains on the other edge of the belt, on the so-called tending side. Said problem can be solved for example by means of the solution explained in the following.

In an embodiment of the invention, the other edge of the polymer- coated belt is thinner in a set area in the lateral direction of the web than the rest of the area in the lateral direction of the belt. Part of the normal thickness of the belt is removed for example by abrading. A suf- ficient change in the thickness can be for example 2 mm. In this em- bodiment, the other edge of the polymer-coated belt is aligned very ac- curately with the edge of the paper web, and in the other edge the sub- stantially thinner edge of the polymer-coated belt remains outside the width of the paper web. Damaging of the section of the polymer-coated belt that remains outside the width of the paper web is, however, pre- vented because in said section the polymer-coated belt is thinner, and thus it does not touch the heated counter surface that functions as a second counter surface of the nip.

The invention can be applied in the shoe calender in such a manner that the belt forming the shell of the shoe roll is oscillated continuously back and forth in the axial direction of the roll in such manner that the roll moves a fixed length of path in the axial direction. The belt of the shoe roll is arranged rotatable by fixing the same from its ends to the end plates of the shoe roll that rotate around a stationary shaft and are journalled on the shaft. The belt of the shoe roll is typically a polymer surface, which is loaded from inside the shell by means of zonally ad- justable loading elements. When the belt of the shoe roll slides in the axial direction of the shoe roll, the edges of the belt are for some time covered by the paper, wherein the thermo roll does not conduct heat on the belt, and, on the other hand, the paper also cools down the belt if the temperature of the belt is higher than that of the paper web.

By means of the shoe calender it is possible to measure the tempera- ture in the edge areas of the belt for example by means of an infrared thermometer and the oscillation is switched on when the temperature

exceeds the set value. It is also possible that the edge of the paper web and the edge of the belt of the shoe roll are aligned with each other on the other edge of the paper web and the temperature of the polymer- coated section of the belt extending over the edge of the paper web in the axial direction of the roll is monitored and said section is cooled down.

In the shoe calender such an arrangement is possible in which both the polymer-coated belt and the gliding shoe and/or roll frame of the shoe roll are moved. It is, for example possible to act in such a manner that the gliding shoe and/or the roll frame of the shoe roll are moved to- wards a so-called driving side, and the polymer-coated belt is moved towards a so-called tending side, in other words, the gliding shoe and/or roll frame of the shoe roll are transferred in opposite directions with respect to each other in the axial direction of the shoe roll. Thus, the other edge of the paper web and the edge of the polymer-coated belt are in alignment with each other, and the other edge of the poly- mer-coated belt is protected by means of the edge area of the above- described belt that is abraded thinner.

In the calender, the transfer of a polymer-coated belt in the transverse direction of the paper web to be treated can also be utilized to prevent the wearing of the polymer-coated belt. If, for example a web treated by means of a zone-controlled roll has to be profiled locally due to a profile error in the paper, this may cause wearing in the belt or local reduction in the surface roughness, which reduces the service life of the belt.

When a polymer-coated belt, such as the belt formed by the shell of the shoe calender or the belt loop of a belt calender is moved in the trans- verse direction of the web to be treated by means of the method ac- cording to the invention, uneven wearing can be reduced.

In the following, the invention will be examined with reference to the appended drawings, in which Figs. 1 and 2 show the principles of some embodiments of the inven- tion, a shoe calender and a belt calender, in a side-view,

Fig. 3 shows a side-view of a nip formed by a shoe roll and a thermo roll, and Fig. 4 shows the cross-section of the edge of the shoe roll on the tending side of the belt.

Fig. 1 shows the principle of a shoe calender in a side-view. The calen- der contains a shoe roll 5, which comprises a rotating belt shell and a stationary central shaft. The belt forming the belt shell can be made of a suitable resilient polymer, such as polyurethane, and inside the belt there is a texture that strengthens the same. The shoe roll 5 is ar- ranged to be loaded zonally by means of loading elements inside the roll. Against the rotating shell of the shoe roll 5 there is a gliding shoe inside the roll, which keeps the belt shell in its shape in a nip N2. The length of the shoe can be even 275 mm. The shoe nip enables the standardization of the nip width irrespective of the loading, roll diame- ters and properties of the belt. This enables the optimization of the variables which are important in the calendering, completely irrespec- tive of each other.

The counter surface of the shoe roll 5 in the nip N2 is a thermo roll 6, which is a hard-faced, heated roll. The surface temperature of the thermo roll that functions as a counter roll for the shoe roll is typically in the order of 80 to 350°. There are auxiliary rolls 7 for the purpose of guiding the paper web W through the nip N2.

In one embodiment of the invention, the belt forming the shell of the shoe roll 5 is moved back and forth in the axial direction of the roll, i. e. in the transverse direction of the paper web W to be treated. Thus, both edges of the belt have to be moved equally in the same direction, so that an axial strain does not occur in the belt. The shoe roll can be made to oscillate continuously for example 50 mm during one minute, and to prevent the wearing of the belt it is possible e. g. to oscillate the polymer surface between the driving side and tending side, in practice for example by means of shoe rolls within a distance of 200 mm, which corresponds to the zone distribution of some multi-zone rolls. The os-

cillation can be implemented for example in such a manner that the set value of an electric belt controller is changed by means of a program approximately 1 mm per hour, wherein the belt slowly moves in the lat- eral direction.

Fig. 2 shows the principle of a belt calender in a side-view. A polymer- coated belt 10 is arranged to rotate around rolls 9, said belt forming a soft counter surface in the nip N3. The paper web W to be treated trav- els between the belt 10 and a hard-faced roll 8. The nip N3 is thus formed by two hard-faced rolls 8, of which at least one is heated, and by a polymer-coated belt 10 travelling therebetween. There are auxil- iary rolls 7 that guide the paper web through the nip N3. In the method according to the invention the belt 10 is moved in the lateral direction, i. e. in the axial direction of the rolls 8 and in the transverse direction of the web W to be treated.

In belt calendering the nip time (nip length) and the nip pressure de- pend on the linear loading and on the coefficient of elasticity of the belt.

The hardness of the belt is typically in the order of 80 to 100 ShA. In the belt calender the length of the nip varies approximately between 10 to 275 mm, and the nip pressure between 5 to 20 MPa.

Fig. 3 illustrates en embodiment of the invention. Fig. 3 shows a nip N2 formed by a shoe roll 5 and a thermo roll 6 in a side-view. When the paper web W to be run is equal in width with the polymer-coated belt 11, the paper web W protects the polymer surface.

When a web W which is narrower than the polymer-coated belt 11 is run in the calender, both the polymer-coated belt 11 and the gliding shoe and/or the roll frame 12 of the shoe roll are transferred. When the web W in question is for example 50 mm narrower than the polymer- coated belt 11, the frame 12 of the roll and the polymer-coated belt 11 are transferred in a manner described hereinbelow. The frame 12 of the roll is transferred 25 mm towards the driving side in the direction of ar- row U by sliding the shaft of the roll on bearings located in the brackets 13 of the shaft. The polymer-coated belt 11, in turn is transferred 25 mm towards the tending side in the direction of arrow S by transferring

the flange 14 of the tending side. A part of the thickness of the polymer- coated belt 11 is abraded off from a fixed area A in the lateral direction of the belt on the edge of the belt on the driving side of the calender (shown in Fig. 4), in the case according to the example from an area of 100 mm in width. The area continues around the entire belt loop in the edge of the belt. The thickness of the abraded layer may be for exam- ple 2 mm, wherein the contact between the thermo roll and the poly- mer-coated roll is prevented.

The invention is not restricted to the description above, but it may vary within the scope of the claims. The means for transferring the polymer surface in the transverse direction of the paper web to be treated can differ from each other. The main aspect of this invention is that the damaging of a polymer-coated belt can be prevented by moving it in the calender in the transverse direction of the web to be treated.