WO2004081282A1

WO2004081282A1 - Calender and method of calendering

Info

Publication number: WO2004081282A1
Application number: PCT/FI2004/000148
Authority: WO
Inventors: Harri Haavisto; Matti Tervonen; Kimmo Vuorinen; Janne Ryhänen; Aaron Mannio
Original assignee: Metso Paper Inc.
Priority date: 2003-03-13
Filing date: 2004-03-15
Publication date: 2004-09-23
Also published as: FI20030377A0; DE112004000421T5

Abstract

The invention relates to a calender (16a) for calendering a paper or paperboard web (10a), the calender (16a) comprising a roll stack of at least three rolls (1a-3a) adapted in a running calender (16a) into a nip contact with each other such that between the superposed rolls are formed nips (N1a, N2a) through which the web (10a) being calendered is arranged to pass. The calender also incorporates an assembly (11a) for transferring at least one of the rolls (1a-3a) in a lateral direction relative to the centerline (9a) of the roll stack.

Description

Calender and method of calendering

The present invention relates to a calender according to the preamble of claim 1 or calendering a web of paper or paperboard.

The invention also relates to a method for calendering a web of paper or paperboard.

The function of calendering is to increase the smoothness and gloss of paper or paperboard, as well as to improve other qualities of the printing surface. In the calen- dering of paper and paperboard, the web is worked in a nip between two opposed rolls. The calender rolls may have a hard or soft surface coating. In soft-surfaced rolls, the coating of the roll is made of paper, other fibrous material or a polymeric material. Hard-surfaced rolls are made in two types: thermorolls and deflection-compensated rolls. Thermorolls are suited for controlling the deformation of the sheet surface. Thermorolls can be heated/cooled by oil, steam or other means such as inductive heating, for instance. Deflection-compensated rolls are suited for controlling the cross-machine thickness and gloss profile of the running web. Conventionally, the shell of deflection-compensated rolls is made of cast iron. In soft-roll and multi-roll calenders, the shell of the deflection-compensated roll in the stack is coated with a soft polymer material.

With increasing calender web speeds, different kinds of complications due to vibrations have emerged more frequently than ever. One of the vibration problems occurring in calendering relates to vertical oscillation of calender stack rolls that causes thickness variations in the web thus degrading the quality of the paper sheet. In the art of papermaking, this phenomenon is known as barring. On the sheet surface, barring appears as uniformly-spaced machine-direction thickness variations that are visible as glazed cross-machine stripes, generally running across the web at a distance of 12 to 250 mm from each other. Barring can deteriorate the printability of, e.g., newsprint, since the printing ink adheres differently on thin and glossy areas of the sheet as compared to thick areas of the sheet. Furthermore, barring increases substantially the maintenance costs of a calender, inasmuch as the calender rolls must be replaced at intervals shorter than normal. If the risk of barring is high, the running speed of the calender and the moisture content of the web must be lowered, whereby the calender throughput falls drastically.

The barring vibration of rolls in the vertical direction is caused by various reasons related to structural factors of the calender, e.g., worn and barred rolls, excitations originating from the wet end of the papermaking machine and vibrations occurring in the press section.

The most general cause of barring, however, is the so-called mechanical feedback effect that invokes self-excitation of calender roll vibrations at the vertical-direction natural vibration modes and natural frequencies of the calender construction. A feedback mechanism can be modeled with the help of a linear spring-mass model, wherein mass elements represent the rolls and the sheet passing via the nips is represented by spring connected between the mass elements when the model simulates the use of hard-surfaced rolls. When a small disturbance sets the calender into vibration at its natural frequency vibration modes, the nip pressure is affected. Herein, after leaving the uppermost calender nip, the paper web has thickness variations that propagate to the subsequent nips and act as an excitation source for vibrations in the nips. If the thickness variations of the sheet enter the next nips in a suitable phase relative to the instantaneous motion of the rolls pressing the web at both sides of a given nip, the amplitude of the natural vibration of the roll pair is amplified. In a running calender, the machine-direction spatial frequency of thickness variations along the traveling sheet is the same as the natural frequency causing the barring thickness variations that invoke the resonance effect. In multi-roll calenders, sheet thickness variations may occur also in other than the uppermost nip meaning that multiple feedback paths may exist in the calender. After an initial excitation, the feedback mechanism needs no further triggering disturbance, but rather, the sheet passing through the nips provides the energy required to maintain the calen- der vibrations. Barring caused by the above-described feedback mechanism occurs particularly in such machine calenders that include nips of hard-surfaced rolls. A multi-roll calender may be a machine calender, super calender or a calender of the OptiLoad/Janus type. In a machine calender, the rolls may have a solid-metal structure. In a supercalender, the rolls are both metal-shell rolls and filled rolls with elastic coating and/or polymer-surfaced rolls in the same fashion as in OptiLoad/Janus- type calenders comprised of polymer-surfaced and metal rolls.

Vibration resulting from the feedback mechanism can be reduced by staggering the calender rolls so that the rolls are individually offset from the centerline of the roll stack, whereby the web travel distance between the successive nips changes. Suitable staggering of the rolls makes it possible to generate between the excitation caused by the thickness variations of the paper sheet and the vertical vibrations of the rolls such a phase shift that attenuates the vibrations excited by the feedback mechanism. Computational models have been developed capable of computing an optimal offset stagger value for each calender roll on the basis of the calender running speed, roll measurement data, web measurement data and roll vibration measurement values.

Subsequently, based the computationally obtained offset stagger values, the rolls are moved laterally by manual means. The rolls are offset, e.g., by placing spacer blocks between the roll mounting point and the calender frame thus forcing the roll to move laterally. Another conventional arrangement comprises moving the calender rolls laterally with the help of a manually operated screw actuator. In both of these constructions, the adjustments are performed in the vicinity of the rolls, whereby the calender for operator safety must be kept stopped over the time the roll offsets are being adjusted. Furthermore, the vertical vibrations of the rolls are not measured in real time in a running calender, but instead, vibration measurements are launched only after sheet barring is probable based on sheet parameter measurements, acoustic emissions or other reasons.

Performing the necessary measurements, determining the optimal offset stagger values and setting the roll offsets manually at the calender rolls is a time-consuming task. Since the shutdown of on-line calenders operating immediately downstream of a papermaking machine is particularly undesirable for such a minor operation as the staggering of the stack rolls, the roll offsets are preferably adjusted only during other production halts. Consequently, as calenders are often run with incorrect stack roll offset stagger settings for very long times up to the next shutdown, the production quality of the calendered sheet is frequently compromised.

It is an object of the present invention to provide a calender construction, wherein the staggering of stack rolls can be performed in a fashion superior to the prior art. The invention also provides an improved assembly construction for the offset stagger adjustment of a calender roll. Moreover, the invention aims to provide an improved method for the offset stagger adjustment of calender rolls.

The goal of the invention is achieved by virtue of laterally offset staggering at least one calender stack roll in regard to the roll stack centerline. In a preferred embodiment of the invention, the vibrations are measured on at least one stack roll in real time in a running calender, whereupon the offset of the roll is adjusted laterally in regard to the roll stack centerline on the basis of the vibration measurement data. Herein, the amplitude and natural frequency values of barring can be monitored in real time in a running calender, whereby the new offset stagger values for the calender rolls may be determined as necessary. Offset stagger adjustment is carried out with the help of actuator means capable of moving the roll without the need for a mechanical task to be performed by the calender operator at the rolls. The stagger adjustment of the calender rolls with the help of a remote-controlled actuator takes place much faster than by manual setting of the roll offset stagger values at the calender roll stack. The roll offset stagger values can be set even during a short production shutdown or even in a running calender. The control of the actuator assembly is carried out manually or automatically with the help of a computer from a control room, for instance. Furthermore, the assembly according to the invention is also easy to mount on existing calenders.

More specifically, the calender according to the invention is characterized by what is stated in the characterizing part of claim 1.

The assembly according to the invention is characterized by what is stated in the characterizing part of claim 9.

Furthermore, the method according to the invention is characterized by what is stated in the characterizing part of claim 11.

5

In the following, the invention is described in more detail by making reference to the appended drawings in which

FIG. 1 shows diagrammatically a side elevation view of one embodiment of a calen- o der according to the invention;

FIG. 2 shows diagrammatically a side elevation view of another embodiment of a calender according to the invention;

5 FIG. 3 shows diagrammatically the operating principle of the invention.

Referring to FIG. 1, a calender 16 shown therein is advantageously a machine calender comprising four hard-surfaced rolls 1, 2, 3 and 4 that in a running calender are in a nip contact with each other so as to form three nips Nl, N2, N3. The calender 16 0 implemented according to the invention typically has 3 to 6 rolls. Of these, rolls 1-4 are generally thermorolls and hard-surfaced deflection-compensated rolls. The lineal load applied at the nips Nl, N2, N3 is determined by both the masses of the rolls in the stack above a given nip and the loading devices of the nips. The lineal loading is generally highest in the lowermost nip. Normally, the lowermost roll 4 of the roll 5 stack is deflection-compensated. When necessary, the nip loading may be additionally increased with the help of hydraulic cylinders pressing the uppermost roll 1 of the roll stack downward, whereby also the uppermost roll 1 is designed deflection-compensated.

0 The shafts of rolls 1-4 are mounted at their both ends in bearing blocks 5, 5'. The bearing blocks 5' of the lowermost roll 4 are fixedly mounted on the calender frame 7. The bearing blocks 5 of the three uppermost rolls 1, 2 and 3 are mounted on sup- port blocks 6 that in turn are slidably mounted on guides 8 of the calender frame 7 or pivotally connected to the calender frame 7. The three uppermost rolls 1-3 are transferred in the vertical direction when the support blocks 6 are moved along the guides 8 or supported by the pivotal connections. 5

The three uppermost rolls 1-3 have their bearing blocks 5 adapted movable along horizontal guides that are provided on the support blocks 6 and are aligned perpendicular to the axes of rolls 1-3 thus allowing the rolls to be transferred laterally along the guides in regard to the centerline 9 of the roll stack. The lateral transfer of the o rolls takes place in a direction perpendicular to the center axis of the roll being moved. The transfer direction of the rolls 1-3 is denoted by arrows 17 in FIG. 1. The centerline 9 of the roll stack denotes a line drawn via the center axes of rolls 1-4 when the rolls 1-4 are in the same center plane.

5 The travel direction of the web 10 being calendered is denoted by an arrow in FIG. 1. The web 10 is introduced first into nip Nl between the first roll 1 uppermost in the calender stack and the second roll 2 of the stack, whereupon the web 10 is passed on the surface the second roll 2 into nip N2 between the second roll 2 and the third roll 3. Subsequently, the web 10 is passed on the surface the third roll 3 into nip N3 0 between the third roll 3 and the fourth roll 4 situated lowermost in the roll stack. After passing through the lowermost nip N3, web 10 is conveyed to the next machine-finishing step downstream of the calender.

Each one of the rolls 1-4 have their bearing blocks 5, 5' equipped with vibration 5 transducers that perform frequency and amplitude measurement of vertical vibrations on rolls 1-4 during the operation of the calender 16. Advantageously, the vibrations of the rolls are measured continuously during the operation of the calender. High vibration amplitudes of rolls 1-4 are generally indicative of barring. Additionally, the frequency of machine-direction thickness variations along the calendered web 10 0 may be recorded with the help of a measurement device 12 such as a thickness gauge beam adapted to operate at a point downstream of the calender. With the help of the measured roll vibration amplitudes and/or the thickness data of the web 10, it is possible to determine the natural frequency or natural frequencies of barring.

When the vibration amplitude at any one of rolls 1-4 grows to a preset limit value, a computational model 13 simulating the vibrations of the calender rolls 1-4 is used to define an optimal offset stagger value for each one of calender rolls 1-3 situated above the lowermost roll 4 of the stack such that sheet barring due to the vibration feedback mechanism can be reduced by using the run-time settings of calender 16. Input data to the computational model are, among others, the running speed of the calender 1 , the temperatures and lineal loads of the nips Nl, N2, N3, the dominant frequencies of vibration and/or the measurement data obtained from the thickness gauging 12 of the calendered web 10. Additional input data to the computational model comprise the basic data of nip rolls in calender 16, such as the diameters, masses and positions of rolls 1-4, complemented with certain characteristic quality values and cross-machine width of web 10 being calendered. Based on the input data, the computational model 13 determines for the three uppermost rolls 1-3 suitable offset stagger values that generate between the excitation caused by the thickness variations of the web 10 and the vertical vibrations of rolls 1-3 such a phase shift that attenuates the vibrations excited by the feedback mechanism.

After the optimal offset stagger values of rolls 1-3 have been determined with the help of the computational model 13, they are compared with the measurement data obtained from sensing the positions of staggerable rolls 1-3. The positions of stagger- able rolls 1-3 relative to the centerline 9 of the roll stack are measured by transducer means such as position or angle transducers having their one part mounted on the bearing blocks 5 while the other part of the transducers is mounted on the support blocks 6. If the offset stagger values YRE_F determined with the help of the computational model 13 differ from the measured offset stagger values Y_m, the rolls 1-3 are moved by the difference E of the values laterally in regard to the centerline 9 of the roll stack. The offset stagger adjustment of rolls 1-3 is performed during a shutdown break of calender 16 or during the operation of the calender 16. The real-time roll offset stagger adjustment in a running calender 16 is possible if the rolls 1-3 are moved sufficiently slowly. The rolls 1-3 are offset staggered with the help of actuators 11, such as hydraulic cylinders, for instance. The first end of actuator 11 is connected to the bearing block 5 of the roll 1-3 to be offset adjusted, while the second end of the actuator is connect- ed to the calender frame 7. To both bearing block 5 of each one of the staggerable rolls 1-3 is connected this kind of actuator 11. When the actuators 11 connected to the calender roll bearing blocks 5 are being operated, the bearing blocks 5 are forced to move along guide rails of the support blocks 6, whereby the roll moves laterally in regard to the roll stack centerline 9 in a direction perpendicular to the center axis of the roll. The actuators may be mounted on each one of the staggerable rolls 1-3 or even only on a given one of the rolls 1-3.

In FIG. 2 is shown another embodiment according to the invention of a calender 16a and assembly 11a for the offset stagger adjustment of the calender stack rolls. The assembly shown herein may also be adapted to operate in the embodiment of FIG. 1. The roll stack of the calender shown in FIG. 2 comprises three hard-surfaced rolls la-3a. Advantageously, the calender 16a is a machine calender wherein the web 10a passes across the distance between the successive nips Nla, N2a entirely on the surface of the intermediate roll 2a. In a running calender 16a, the rolls la-3a are in a nip contact with each other thus forming two nips Nla, N2a. The travel direction of the web 10a being calendered is denoted by an arrow in FIG. 2. The web 10a is first introduced into nip Nla between the uppermost roll la of the roll stack and the intermediate roll 2a of the roll stack, whereupon the web 10a is passed on the surface of the intermediate roll 2a into the lower nip N2a between the intermediate roll 2a and the third roll 3a. After passing through the lower nip N2a, the web 10a travels to the next machine finishing step.

The shafts of rolls la-3a are supported at both roll ends on bearing blocks 5a, 5 'a. The bearing blocks 5 'a of the lowermost roll 3a are fixedly mounted on the calender frame 7a. The bearing blocks 5a of the uppermost roll la are connected to support blocks 6'a that in turn are slidably mounted on guides 8a of the calender frame 7a. Alternatively, the support blocks 6'a of the uppermost roll la may be pivotally mounted on the calender frame 7a. The bearing blocks 5 a of the staggerable roll 2a, that is, of the intermediate roll 2a are mounted on loading arms 18a that are further connected to support blocks 6a. The support blocks 6a are slidably mounted on guides 8a of calender frame 7a. Alternatively, the support blocks 6a may be pivotally mounted on the frame 7a of calender 16a with the help of pivotal arms whose pivot axes are aligned parallel to the axis of the intermediate roll 2a. Thus, the two uppermost rolls la, 2a are actuated to move in the vertical direction when the support blocks 6'a, 6a are transferred along guides 8a or the support blocks are rotated about their pivotal point.

To either one of bearing blocks 5a of intermediate roll 2a is connected an actuator 11a capable of transferring the intermediate roll 2a laterally in regard to the roll stack centerline 9a. The transfer direction is perpendicular to the center axis of the intermediate roll 2a. Herein, roll stack centerline 9a refers to a line passing via the center axes of the rolls la-3a when the rolls 1-4 are in the same center plane. The lateral transfer direction of the intermediate roll 2a is denoted in the diagram by arrow 17a. The actuator 11a comprises a loading arm 18a connected by its first end to the bearing block 5a. The loading arm 18a is rigidly mounted on the bearing block 5a with screws, for instance. The other end of the loading arm 18a is connected to the calender frame 7a so that the loading arm 18a can move in the transfer direction 17a of the intermediate roll 2a. Between the second end of the loading arm 18a and the calender frame 7a may be adapted load-relief means, such as load-relief bellows, suitable for pressing one end of the loading arm 18a downward, whereby the nip load between the lower roll 3 a and the intermediate roll 2a is reduced.

The construction of actuator 11a additionally comprises, connected to the support block 6a of the intermediate roll 2a, a stiff pivotally jointed lever arm 19a with a first end and a second end. The first end of lever arm 19a is pivotally 20a connected on the support block 6a. The pivot shaft 20a is aligned parallel to the center axis of the intermediate roll 2a. At a point between its first and second ends, the pivotal lever arm 19a is mounted rigidly on the loading arm 18a. This mounting point 21a of the pivotal lever arm 19a to the loading arm 18a is adapted to the same height level as the center axis of the intermediate roll 2a in the transfer direction 17a of the intermediate roll 2a. To the second end 22a of the pivotal lever arm 19a are connected actuator means serving to the rotate the pivotal lever arm 19a about the pivot shaft 20a.

The actuator of pivotal lever arm 19a may be a screw actuator, for instance. The screw 23 a of the actuator is threaded. On the support block 6a is mounted a bracket 26a with a hole having the first end of the actuator screw 23 a fitted therein. To the second end 22a of the pivotal lever arm 19a is made a threaded hole having the actuator screw 23 a screwed therein. The second end 22a of pivotal lever arm 19a also includes a support bracket 24a having the drive means actuating the screw 23 a mounted thereon. The screw drive means comprises a motor with a gear wheel adapted mating with the threads of screw 23 a. When the gear wheel is driven by the motor, screw 23a is set in rotation. Screw 23a is adapted into the hole of bracket 26a so that no axial movement of the screw is possible during its rotation. Thus, the rotation of screw 23 a forces the second end 22a of the pivotal lever arm 19a to move in the axial direction of screw 23 a. As a result, the pivotal lever arm 19a rotates about its pivotal shaft 20a, whereby the loading arm 18a moves and transfers the intermediate roll 2a laterally in regard to the center line 9a of the calender roll stack so that the direction of the transfer movement is perpendicular to the center axis of the intermediate roll 2a. Operable in conjunction with screw 23 a is adapted a transducer device suited for measuring the actuated number of turns of screw 23 a.

The stroke of actuator 1 la is dimensioned so that it facilitates the movement of the intermediate roll 2a by max. 70 mm laterally in regard to the centerline 9a of the roll stack. Advantageously, the intermediate roll 2a is adapted transferable to both sides of the roll stack centerline 9a, i.e., to the side of the guides 8a located closer to the frame of the calender 16a and to the opposite side in regard to the guides 8a. Hence, the transfer movement may extend by 35 mm, for instance, to both sides of the center line 9a.

To the bearing blocks 5a, 5 'a of certain ones of the calender rolls or, alternatively, to each one of rolls la-3a are attached vibration transducers 25a capable of measuring the frequencies and amplitudes of the vertical vibrations of rolls la-3a during the operation of the calender 16a. Advantageously, the vibration transducers are attached at least on the bearing blocks of the laterally transferable roll 2a. Further advantageously, the roll vibrations are continuously measured in the running calender. As a rule, intensive vibrations of rolls la-3a are indicative of sheet barring. Additionally, the spatial frequency of machine-direction thickness variations of the calendered web 10a can be measured by means of a measurement device 12a, such as a thickness gauge beam, located downstream of the calender. On the basis of the measured roll vibration frequencies and/or thickness variation data of the web 10a, it is then possible to determine the natural frequency or natural frequencies at which barring occurs.

When the vibration amplitude of rolls la-3a grows to a preset limit value, a computa- tional model 13 capable simulating the vibrations of rolls la-3a is used to define an optimal offset stagger value for the intermediate roll 2a such that sheet barring due to the vibration feedback mechanism can be reduced at the instantaneous running settings of calender 16a. Input data to the computational model are, among others, the ninning speed of the calender 16a, the temperatures and lineal loads of the nips Nla and N2a, the dominant natural frequency components of roll vibrations and/or the measurement data obtained from the thickness gauging 12a of the calendered web 10a. The computational model additionally utilizes the data of the rolls of calender 16a, such as the diameters, masses and positions of rolls la-3a, complemented with the characteristic qualities and cross-machine width of the web 10a being calendered. Based on these input data, the computational model 13a determines for the intermediate roll 2a a suitable offset stagger value that generates between the excitation caused by the thickness variations of the web 10a and the vertical vibrations of rolls la-3a such a phase shift that attenuates the vibrations excited by the feedback mechanism.

After the optimal offset stagger value of the intermediate roll 2a has been determined with the help of the computational model 13, this optimal offset stagger value is compared with the measurement data obtained from sensing the position of roll 2a. The position of the staggerable roll 2a relative to the centerline 9 of the roll stack is measured by transducer means such as position or angle transducers having their one part mounted on the loading arms 18a of the intermediate roll 2a while the other part of the transducers is mounted on the support blocks 6a. If the offset stagger values YRE_F determined with the help of the computational model 13 differ from the measured actual offset stagger values Y_m, the intermediate roll 2a is moved laterally by the difference E of the values in regard to the centerline 9a of the roll stacks. The offset stagger adjustment of the intermediate roll 2a is performed during a shutdown break of calender 16a or on-line in a running calender 16a. The on-line roll offset stagger adjustment in a running calender 16a is possible if the intermediate roll 2a is moved sufficiently slowly.

The actuators 1 la move the intermediate roll 2a by the difference E between the measured actual offset stagger value Y_m and the offset stagger value Y_REF determined with the help of the computational model. The roll offset stagger actuators 1 la are controlled, e.g., manually from the control room or with the help of a computer. Computer-controlled operation is particularly advantageous when the offset stagger value of the intermediate roll 2a is set in a running calender 16a. After the inter- mediate roll 2a has been transferred into the optimal offset stagger position YREF determined with the help of the computational model, a new optimal position is computed for the intermediate roll 2a if the vibration amplitude of rolls la-3a exceeds the preset limit value. Subsequently, the offset stagger position of the intermediate roll 2a is readjusted during a production shutdown or in the running calender 16a. Thus, a feedback control system can be utilized in the offset stagger adjustment of the calender rolls.

FIG. 3 shows schematically the operating principle of the invention. The vibrations of the stack rolls and the frequency of machine-direction thickness variations of the calendered web are recorded 27 in a running calender 16; 16a. The dominant frequencies are determined from the vibration measurement signals with the help of spectral analysis, for instance. When the amplitude values of roll vibrations exceed a preset level, the computational model 13 of the roll vibrations having its input data based on the roll vibration values, thickness variations of the web and the calender operational data allows the optimal lateral offset stagger values Y_REF of the calender rolls relative to roll stack centerline to be determined. The positions of the staggera- ble rolls are continuously monitored in a running calender with the help of position transducers. With the help of the actuators 11; 11a driven by a control unit 15, the offset values of the staggerable rolls are then changed by the difference E between the measured offset position Y of a given roll and the optimal offset stagger value Y_REF obtained from the computational model for the given roll. At the instant the value of difference E is zero, the staggerable rolls are known to be driven into their proper positions corresponding to the optimal offset stagger values YREF, whereby the staggering movement of the rolls can be stopped. As the vibration amplitude of the stack rolls next time grows above the preset limit value, new optimizing roll offset values are requested from the computational model 13 and the offset stagger positions of the stack rolls are readjusted.

The invention may also be implemented using embodiments different from those described above.

In the above-described embodiments, the lowemiost roll 4; 3 a of the calender is mounted fixedly. Alternatively, the fixedly mounted roll may be other than the lowermost roll 4, for instance, roll 3 located next above the lowermost roll. However, generally only one of the rolls in a machine calender is fixedly mounted in the calender frame.

Should the calender stack comprises three rolls only, the actuators 11a may be adapted on the intermediate roll alone in the fashion of the embodiment shown in FIG. 2, whereby the transfer of the intermediate roll allows the web travel distance to be changed between the two calender nips. In a calender stack of six rolls, the actua- tors 11a can be adapted to operate only on the roll next below the uppermost roll and on the roll next above the lowermost roll. Herein, the web travel distance between each nip of the calender can be altered by moving two rolls of the stack. The invention is adaptable to all types of multiroll calender constructions. Although the preferred embodiment of the invention is related to a calender having its entire roll stack comprised of hard-surfaced rolls, the invention may as well be implement- ed in calenders comprising soft-surfaced rolls, such as rolls with a fibrous or polymer material coating.

Furthermore, the position measurement of the staggerable rolls may be carried out with the help of a position transducer connected to the actuator 11; 1 la in such a fashion that the roll position Y_M is determined from the movement of the actuator such as a hydraulic cylinder or a screw actuator.

Claims

Claims:

1. A calender (16; 16a) for calendering a paper or paperboard web (10; 10a), the calender (16; 16a) comprising

a roll stack of at least three rolls (1-4; la-3a) adapted in a running calender (16; 16a) into a nip contact with each other such that between the superposed rolls are formed nips (Nl, N2, N3; Nla, N2a) through which a web (10; 10a) being calendered is arranged to pass,

characterized by

an assembly (11; 11a) for transferring at least one of said rolls (1 a-3a; 1-4) in a lateral direction relative to the centerline (9; 9a) of said roll stack.

2. The calender of claim 1 having the transferable roll (2a) mounted at its both ends in bearing blocks (5a), characterised in that said assembly (11a) for transferring said roll (2a) comprises - a loading arm (18a) connected to said bearing block (5a) of said transferable roll (2a), a pivotal lever arm (19a) having a first end and a second end (22a), the pivotal lever arm (19a) being pivotally (20a) connected by its first end to a support block (6a) mounted on the calender frame (7a) and further being connected at a point between its first end and its second end to a loading arm (18a), and actuator means (23 a) acting on the second end (22a) of said pivotal lever arm (19a) so as to rotate said pivotal lever arm (19a) about said pivot point (20a).

3. The calender of claim 2, characterized in that said actuator means comprise a threaded screw (23 a) connected to said second end (22a) of said pivotal lever arm (19a) and actuator means for rotating said screw (23 a).

4. The calender of any one of foregoing claims, characterized by means (25a) for measuring the vibrations of at least one of said rolls (la-3a; 1-4).

5. The calender of claim 4, characterized by a computational model (13) simulating the behavior of said calender (16; 16a) for the determination of optimal values (YR_EF) of the offset displacement of the staggerable calender rolls relative to the centerline (9; 9a) of the roll stack, said computational model (13) being adapted to be activated at the instant the vibration amplitude values of calender rolls (1-4; la-3a) grow above a preset limit value.

6. The calender of any one of foregoing claims, characterized in that all of the rolls of said roll stack are hard-surfaced rolls.

7. The calender of any one of foregoing claims, characterized in that the means (11; 11 a) for transferring a roll are adapted to function in a running calender (16; 16a).

8. The calender of any one of foregoing claims, characterized by means (12;

12a) for gauging machine-direction thickness variations of the calendered web (10; 10a) in a running calender (16; 16a).

9. An assembly (11a) for transferring a calender roll (2a) in a lateral direction relative to the centerline (9a) of a roll stack, characterized by a loading arm (18a) connectable to a bearing block (5a) of said transferable roll (2a), a pivotal lever arm (19a) having a first end and a second end (22a), the pivotal lever arm (19a) being pivotally (20a) connectable by its first end to a support block (6a) mounted on the calender frame (7a) and further being connected at a point between its first end and its second end to a loading arm (18a), and actuator means acting on the second end (22a) of said pivotal lever arm (19a) so as to rotate said pivotal lever arm (19a) about said pivot point (20a).

10. The assembly of claim 9, characterized in that said actuator means comprise a threaded screw (23 a) connected to said second end of said pivotal lever arm (19a) and actuator means for rotating said screw (23a).

11. A method for calendering a paper or paperboard web (10; 10a) in a calender (16; 16a), the calender (16; 16a) comprising a roll stack of at least three rolls

(1-4; la-3a) adapted in a running calender (16; 16a) into a nip contact with each other such that between the superposed rolls are formed nips (Nl, N2, N3; Nla, N2a) through which the web (10; 10a) being calendered is arranged to pass, characterized in that

at least one of the rolls is transferred in a lateral direction relative to the centerline (9; 9a) of said roll stack.

12. The method of claim 11, characterised in that said roll is transferred relative to the centerline (9; 9a) of said roll stack by the actuator means of claim 8 or 9.

13. The method of claim 11 or 12, characterized in that the vibrations of at least one of said rolls (la-3a; 1-4) are measured in a running calender (16; 16a).

14. The method of claim 13 , characterized by the step of determining the optimal value (Y_REF) of the lateral offset displacement of the staggerable calender roll relative to the centerline (9; 9a) of the roll stack on the basis of a vibration measurement, whereupon said roll is transferred to its optimal offset stagger position (YREF).

15. The method of any one of foregoing claims, characterized in that the offset stagger positions of the rolls are altered in a running calender (16; 16a).

16. The method of any one of foregoing claims, characterized in that the machine-direction thickness variations of the calendered web (10; 10a) are measured and the dominant natural frequencies of the web thickness variations

5 are determined in a running calender (16; 16a).

17. The method of claim 16, characterized in that the optimal offset stagger values of staggerable rolls are determined based on the running settings and vibration measurement signals of the calender (16; 16a) and the gauging data o of the thickness variations of the web (10; 10a) being calendered.