The present invention relates to an improved calender for calendering a web, in particular a calender according to the preamble of claim 1.

During the 1990's a new calendering technique has been introduced, the extended soft nip or the LN-calender (i. e. Long Nip) (Arnesjo P., Håkansson S., Tuomisto M. and. Tani M. T (1999) :"Korsnas AB, Gavle, Sweden: The first mill to operate a shoe calender", Proc. TAPPI Papermakers Conference, TAPPI PRESS, Atlanta, USA, p. 765.) where the advantages of soft calendering were further developed.

Compared to conventional soft nips, the local stress concentrations in the calender nip are substantially reduced with the extended soft nip. The required smooting of the paper surface can therefore be obtained with a minor or no increase of the lo- cal variations of the surface properties (Wikstrom M., Nylund T. and Rigdahl, M.

(1997) :"Calendering of coated paper and board in an extended soft nip", Nordic Pulp Paper Res. J., 12: 4,289).

The LN-concept has been tested in different configurations; the shoe-belt and the roll-belt, see FIGS. 1A, 1B and 1C.

The shoe-belt configuration 10 as shown in FIG. lA consists of a stationary hy- draulically loaded concave press shoe 12 (as shown in FIG. 1C) and an endless polymer belt 14. To prevent the friction heat developed between the stationary press shoe and the mobile polymer belt from becoming too high, an intermediate layer in the form of an oil film 16, which dissipates the pressure force, is used as shown in FIG. 1C. The length and shape of the press shoe is the dominating fac- tors determining the nip length.

The roll-belt configuration 18 shown in FIG. 1B, is the second type of extended soft calender nip used. For this configuration, a rotary steel roll 20 instead of a stationary press shoe supports the polymer belt 22. The extended nip length is determined mainly by the belt thickness and the compressive deformation behav- ior of the polymer belt, which is significantly more deformable than a conven- tional backing roll cover. A roll that stretches the belt and an alignment roll that controls the CD (Cross machine Direction)-position of the belt are other neces- sary components of this configuration. The static nip length is estimated to be about 20-35 mm with the roll-belt configuration.

For some years both configurations were promoted (Tani M. and Hiirsalrni L (1998):"New bulk preserving calendering method based on long nip", Proc. 2nd ECOPAPERTECH Conf., Helsinki, Finland p. 137). However, nowadays only the shoe-belt configuration 10 is marketed (Turtinen P. and Tani M. (1998): "OptiDwell-The new bulk preserving calendering method", PITA Ann. Conf., p.

53). The reason for this is due to difficulties with scaling up of the roll-belt con- figuration 18, for example difficulties in controlling the cross machine direction alignment of the polymer belt 22. Nevertheless the roll-belt configuration 18 has its advantages compared to the shoe-belt configuration: * Due to the fact that a rotary steel roll is used, instead of a lubricated stationary press shoe to support the polymer belt, a higher maximum compressive stress can be applied. This is beneficial when aiming for higher degrees of surface deformation of the paper products.

* The construction is simplified (and made possibly more cost effective) by avoiding the shoe press technology.

'The energy consumption is most likely lower compared to the shoe-belt con- figuration since the nip is obtained without a stationary support, thus the mo- mentum, i. e. the friction, which consumes power will be lowered.

It is an object of the present invention to provide a soft nip calender of simpler design.

It is a further object of the present invention to provide a soft nip calender which provides greater maximum compressive forces.

It is another object of the present invention to provide a soft nip calender which is more energy-efficient.

Further objects, features and advantages of the invention will be apparent from the following description. The calender according to the invention is mainly charac- terized in that which is set forth in the characterizing part of claim 1.

The calender of this invention has a heated first roll, mounted for rotation in a calender frame, which is opposed to and forms a nip with a soft surface counter roll system. The soft surface counter roll system is comprised of a second roll which has an outer shell. The second roll is mounted for rotation to the calender frame. An endless polyurethane belt or blanket having an inside diameter ap- proximately 10 to 20 mm larger than the outside diameter of the second roll shell is mounted about the second roll. The outer circumferential edges of the polyu- rethane belt are held by clamping rings to bearings which are circumferentially mounted about the second roll on the roll gables. A small quantity of oil may be placed between the surface of the second roller shell and the polyurethane belt.

FIG. 1A is a side elevational view of a prior art long nip calender of the shoe-belt type.

FIG. 1B is a side elevational view of a prior art long nip calender of the lubricated roll belt type.

FIG. 1 C is a detail view of the lubricating system for the prior art calender of FIG.

1A.

FIG. 2 is an isometric view, partially cut away in section, of the long nip calender of this invention.

FIG. 3A is a side elevational schematic drawing of the long nip calender of FIG.

2.

FIG. 3B is in an enlarged fragmentary view of the long nip calender of FIG. 3A.

FIG. 4 is a cross-sectional view of the long nip calender of FIG. 2 taken along section line 4-4.

In the figures following reference numbers are used: 10 shoe-belt configuration 12 hydraulically loaded concave press shoe 14 endless polymer belt 16 oil film 18 roll-belt configuration 20 rotary steel roll 22 polymer belt 24 new concept for LN-calender 26 web 28 first calendering roll 30 surface 32 roller shell 34 soft surface counter roll system 36 calendering nip

38 smooth polymer mantle belt 40 rotary calender shell 41 second calendar roll 42 inner diameter 44 outer diameter 46 gable 48 clamping ring 50 bearings 52 chamber 53 roll neck 54 roll bearing 56 calender frame 58 roll neck 60 roll bearing Referring more particularly to FIGS. 1A-4 wherein like numbers refer to similar parts, a new concept for LN-calender 24 is proposed in which the same order of compressive stress can be applied as with the"traditional roll-belt configuration".

At the same time it takes care of the difficulties with the traditional roll-belt con- figuration.

Specifically, the invention provides a calender 24 for calendering a web 26, such as a paper web, coated paper web, or paper board web. The LN-calender 24 has a rotating and heatable first calendering roll 28. The surface 30 of the first roll shell 32 which comes into contact with the web 26 has a hard and smooth calendering surface 30. A rotary soft surface counter roll system 34 cooperates with the heat- able calendering roll 28 for defining therebetween a calendering nip 36 for the passage therethrough of the web 26. The soft surface counter roll system 34 com- prises a smooth polymer mantle belt 38 on the surface of a rotary second calender shell 40 of the second calender roll 41. The inner diameter 42 of the mantle belt 38 is 5-50 mm larger than the outer diameter 44 of the second roll shell 40.

In one embodiment of the invention, the polymer mantle belt 38 is joined to and positioned with respect to the gable 46 of the roll 41 by means of a clamping ring 48, which is preferably attached to the roller gable 46 by bearings 50.

In a preferred embodiment of the invention the inner diameter of the mantle belt 38 is 10-20 mm larger than the outer diameter 44 of the roll shell 40.

The polymer of the mantle belt 38 in a particularly preferred embodiment is polyurethane.

In a further embodiment of the invention there is an oil film between the polymer mantle belt 38 and the rotary calender shell 40.

The invention replaces the stationary beam and shoe system of the shoe-belt con- figuration 10 with a rotary steel roll. The belt 38 is joined and positioned onto the roller gables 46 by the same type of clamping ring as used for the shoe-belt con- figuration 10. The inner diameter of the endless mantle belt 38 is approximately 5-50 mm, preferably 10-20 mm larger than the outer diameter of the roll shell 40 in order to avoid unnecessary shear forces between the steel roll 41 and the belt 38 and, moreover, to facilitate change of the belt 38. In order to allow speed dif- ferences between the belt 38 and the steel roll 41, the clamping rings 48 are at- tached to the steel roll by bearings 50. Due to that the clamping rings 48 can rotate independently of the roll shell 40.

The first calender roll 28 has a roll neck 53 by which the roll 28 is mounted by a roll bearing 54 to a calender frame 56. Similarly, the second calender roll 41 has a roll neck 58 which is mounted by a roll bearing 60 to the frame 56.

The new concept solves most of the difficulties with the traditional roll-belt con- figuration 18:

* The clamping rings 48 take care of the difficulties in controlling the cross machine direction alignment of the polymer belt. This is proven to work for a shoe-belt configuration exceeding a width of seven meters.

* The system with the clamping ring 48 on the roller gables 50 provides a closed volume 52 with the belt 38, thus the pollution problem is solved.

* The cord of the base fabric of the belt can be arranged to reward expansion in machine direction (MD), but not in the cross machine direction (CD).

In addition, if it is necessary to allow the belt 38 to expand in the cross machine direction to some extent, a limited proportion of oil can be added to the chamber 52 formed between the roll shell and the belt. An oil film will than provide hydro- dynamic lubrication between the roll shell 40 and the belt 38 in the nip 36.

It is understood that the invention is not limited to the particular construction and arrangement of parts herein illustrated and described.