EP1422342A1 - Method and arrangement for computing and regulation of the distribution of linear load in a multi-nip calender and a multi-nip calender - Google Patents

Abstract

The invention concerns a method and an arrangement forcomputing and regulation of the distribution of linear loadin a multi-nip calender. The material web (W) is passedthrough the nips (N1...N9) in the set of rolls (12), whichset of rolls comprises a variable-crown upper roll (13), avariable-crown lower roll (14) and intermediate rolls(15...22) fitted between the upper and lower rolls (13,14). All the rolls in the set of rolls are supported sothat, when the nips (N1...N9) are closed, the bending linesof the rolls are curved downwards. In the computing andregulation of linear loads, the physical propertiesaffecting the bending of each intermediate roll (15...22)under load, such as bending rigidity, mass, shape, andmaterial properties, are taken into account. The ratio ofthe linear loads applied to the intermediate rolls(15...22), the own weight of the rolls, and of the supportforces applied to the rolls is regulated so that the set ofrolls is in a state of equilibrium and in a predeterminedstate of deflection. The invention also concerns a multi-nipcalender that carries out the method.

Description

• The invention concerns a method for computing and regulation of the distribution oflinear load in a multi-nip calender, wherein the material web to be calendered ispassed through the nips in a set of rolls that is placed in a substantially verticalposition, which set of rolls is formed by a variable-crown upper roll, a variable-crownlower roll and by at least two intermediate rolls provided with supportcylinders and fitted between the upper and lower rolls, in which connection all therolls in the set of rolls are supported so that, when the nips are closed, the bendinglines of the rolls are curved downwards.
• Further, the invention concerns an arrangement for computing and regulation of thedistribution of linear load in a multi-nip calender meant for calendering of paper orboard, which calender comprises a set of rolls which is mounted on the frame of thecalender in a substantially vertical position and which set of rolls includes a variable-crownupper roll, a variable-crown lower roll as well as one or several intermediaterolls fitted between the upper roll and the lower roll, in which connection the meansof suspension of the intermediate rolls are provided with support cylinders, and allthe rolls in the set of rolls are supported so that, when the nips are closed, thebending lines of the rolls are curved downwards.
• Further, the invention concerns a multi-nip calender for carrying out the method inaccordance with the invention.
• In conventional supercalenders, when the nips are closed, the set of rolls is supportedfrom outside the zone of treatment of the web by means of forces which aresubstantially equal to the what is called pin load applied to the bearing housings ofthe rolls during running, or which are lower than said pin load. The pin load is commonly defined so that it includes the weight of all of the auxiliary equipmentconnected with the bearing housings of the roll, such as gap shields, doctors, and so-calledtake-out leading rolls, and also the weight of the portion placed outside theweb width and the weight of the bearing system. This prior art has been describedbest in the paper byRolf van Haag: "Der Weg zum Load Control-System "; DasPapier, 1990, Heft 7, in which the regulation of the linear load in a conventionalsupercalender is described. In these calenders, the rolls are positioned one above theother so that their middle portions are curved upwards or, in a very rare specialcase, are fully straight. The intermediate rolls do not bend in the same way, ascompared with one another. Owing to the mode of running, the nip loads in the setof calender rolls are always such that the roll masses occurring in the area of theweb to be calendered always act with full effect upon all the nip loads placedunderneath the roll concerned. In such a mode of running, it is assumed that the setof rolls is curved in such a way during running that the rigidities of the rolls do nothave a substantial effect on the uniformity of the linear loads, and attempts are madeto operate the calender based on this assumption so that exclusively the linear loadsof the upper roll and of the lower roll are regulated on the basis of measurements ofquality.
• In theFinnish Patent No. 96, 334 and in the equivalentUS Patent No. 5,438,920, acalendering method and a calender that applies the method have been described,which calender comprises a variable-crown upper roll, a variable-crown lower rolland a number of intermediate rolls placed between the upper roll and the lower rollin nip contact with each other, said rolls having been arranged as a substantiallyvertical stack of rolls on the frame of the calender, the material web to be calenderedbeing passed through said nips. In said patents, an idea was suggestedaccording to which the nip load produced by the masses of the rolls in the stack ofrolls was eliminated in the desired way so that all the nips in the calender could beloaded with the desired load, which load was, in a preferred alternative embodiment,equally high in all nips. Thus, the calendering potential could be utilized substantiallybetter than in the earlier calenders. It was one of the basic ideas in said prior-artcalender that rolls bending in the same way were employed in the calender. In said publications, the conduct of such substantially equally bending rolls in thecalender and the simple possibility, permitted by said rolls, of relieving the wholemass of the roll were described, in which case said prior-art calender and calenderingmethod differ essentially from the first-mentioned German prior art in the veryrespect that the effect of the masses of the rolls on the linear loads in the lower nipscan be regulated freely.
• The prior art described above involves an essential problem. If it is assumed that thenatural deflections of the intermediate rolls in the calender without linear loads, i.e.when the nips are open, and the rigidities of the rolls as well as the masses aredifferent, first it is to be stated that such rolls do not comply with those described insaidFI Patent 96, 334 orUS Patent 5, 438, 920, in which patents all of the intermediaterolls had substantially equal deflections. In reality, the manufacture of such rolls,which substantially meet the absolute requirement stated in said publications withoutseparate operations, is very difficult and also expensive, in which connection it hasbeen ascertained that an entirely trivial algorithm of regulation of linear loads, whichdoes not take into account minor differences between the rolls, is not adequate fromthe point of view of reliable operation of the calender.
• The object of the present invention is to provide a solution for the problems relatedto prior art by developing a novel mode of thinking, which takes into account theproperties of deflection of the rolls. One object is to provide an improvement overthe calender concept described in theFI Patent 96, 334 andUS Patent 5,438,920, inparticular in respect of the way in which the distribution of linear load can bebrought under control in the desired way.
• In view of achieving this object, the method in accordance with the invention ismainly characterized in that, in the computing and regulation of linear loads, thephysical properties affecting the bending of each intermediate roll under load, suchas bending rigidity, mass, shape, and material properties, are taken into account, andthe ratio of the linear loads applied to the intermediate rolls, the own weight of the rolls, and of the support forces applied to the rolls is regulated so that the set of rollsis in a state of equilibrium and in a predetermined state of deflection.
• On the other hand, the arrangement in accordance with the invention is mainlycharacterized in that the arrangement includes an automation system and a computingunit, which have been fitted, in the computing and regulation of linear loads, to takeinto account the physical properties affecting the bending of each intermediate rollunder load, such as bending rigidity, mass, shape, and material properties, and toregulate the ratio of the linear loads applied to the intermediate rolls, the own weightof the rolls, and of the support forces applied to the rolls so that the set of rolls isin a state of equilibrium and in a predetermined state of deflection.
• The method in accordance with the invention takes into account the properties ofrolls of all types, and, thus, in an embodiment of the invention, in the method, inthe set of rolls in the calender, intermediate rolls are employed whose bendingproperties are different from roll to roll.
• In the computing in accordance with the method and the arrangement, the set of rollscan be treated as a single unit. On the other hand, the computing can also be carriedout individually in respect of each pair of rolls.
• The intermediate rolls in the set of rolls are freely moving, so that just forces areapplied to the rolls, but the rolls are not held in position.
• By means of the method and the arrangement in accordance with the invention andby means of the calender intended for carrying out the method, significant advantagesare obtained in particular in the respect that, by means of the arrangement inaccordance with the invention, the linear loads in each nip can be regulated to thedesired level. The arrangement takes into account and computes the deflection linesof the intermediate rolls and the loads of the relief cylinders corresponding to saidlines. The rigidities of the intermediate rolls and the differences in the naturaldeflections of the rolls arising from differences in mass can be compensated for readily in the arrangement by regulating the support forces of the roll supportcylinders. Thus, when an arrangement in accordance with the present invention isemployed, the deflection lines of all of the intermediate rolls do not have to beidentical. The method and the arrangement of the invention can be applied both witha traditional mode of running of a multi-nip calender, in which the paper web runsthrough all nips, and to a modified mode of running, in which the paper web ispassed through certain, desired nips only. Further advantages and characteristicfeatures of the invention will come out better from the following detailed descriptionof the invention.
• In the following, the invention will be described by way of example with referenceto the figures in the accompanying drawing.
• Figure 1 is a general illustration of the arrangement in accordance with the inventionwhich is applied in a multi-nip calender for computing and regulation of the distributionof linear load.
• Figures 2A, 2B and 2C are exemplifying illustrations of the sorts of regulation of thedistribution of linear load in the machine direction that can be achieved by means ofthe arrangement in accordance with the invention.
• Figures 3A, 3B and 3C illustrate the effects of different calendering parameters onthe surface properties of paper.
• Figure 4 is a schematic illustration of the relative arrangement of the data basesincluded in the automation arrangement in accordance with the invention.
• Figure 5 is a schematic illustration of a four-roll calender that carries into effect themethod in accordance with the invention.
• Figure 6 is a schematic illustration of an alternative mode of loading in a multi-rollcalender in which the set of rolls in the calender is treated by pairs of rolls.
• Figures 7A, 7B and 7C are schematic side views illustrating alternative embodimentsof the set of rolls in a multi-roll calender in which a mode of loading described inrelation to Fig. 6 is employed.
• Figure 8 shows a schematic block diagram that illustrates a model of computing inthe arrangement in accordance with the invention.
• Thus, Fig. 1 is a general view of the arrangement in accordance with the invention,and in this figure the calender is denoted generally with the reference numeral 10,the automation system included in the invention with the reference numeral 30, andthe computing unit included in the automation system with the reference numeral 40.The calender 10 shown in Fig. 1 has a construction similar to that described, e.g.,in theFI Patent 96, 334, and, thus, the calender comprises a calender frame 11, onwhich the set of rolls 12 consisting of a number of rolls has been installed substantiallyin the vertical plane. The set of rolls 12 comprises an upper roll 13, a lowerroll 14, and a number of intermediate rolls 15...22 fitted between the upper roll andthe lower roll one above the other, which rolls are, in the situation illustrated in Fig.1, in nip contact with each other. The paper web W is passed over alignment,spreader and take-out leading rolls into the upper nip N₁ and further through theother nips N₂... N₈ in the calender and finally out through the lower nip N₉. In theway illustrated in Fig. 1, the paper web W is taken, in the gaps between the nipsN₁ ... N₉, apart from the faces of the calender rolls by means of take-out leadingrolls.
• The upper roll 13 in the calender is a variable-crown roll, for example a roll adjustablein zones, whose bearing housing 131 has been attached directly to the calenderframe 11. The axle of the variable-crown upper roll 13 has been mounted in saidbearing housing 131, and, in the normal way, the roll is provided with inside loadingmeans, for example zone cylinders, by whose means the deflection of the roll mantlecan be regulated in the desired way.
• In a similar way, the lower roll 14 in the calender is a variable-crown roll, inparticular a roll adjustable in zones, whose mantle has been mounted revolving onthe roll axle and which roll 14 is provided with inner loading means, for examplezone cylinders, by whose means the deflection of the roll mantle can be regulated inthe desired way. The axle of the lower roll 14 has been mounted in bearing housings141, which have been mounted, in the way shown in Fig. 1, on loading arms 142,which have been attached to the calender frame pivotally by means of articulatedjoints 143. Between the calender frame 11 and the loading arms 142, lower cylinders144 have been mounted, by whose means the lower roll 14 can be shifted in thevertical plane. Thus, the set of rolls 12 can be loaded by means of the lower cylinders144, and, further, by means of said lower cylinders 144, if necessary, it ispossible to open the set of rolls 12. By means of the zone cylinders of the variable-crownupper and lower rolls 13, 14, in the method and the arrangement in accordancewith the invention, a necessary correction and regulation of the cross-directionprofile of the paper web W can be carried out.
• Between the upper and the lower rolls 13,14 in the calender, a number of intermediaterolls 15...22, which are in nip contact with each other, have been fitted, as wasalready stated above. In the following, exclusively the topmost intermediate roll 15will be examined, and the related constructions are described in more detail with theaid of reference numerals. A corresponding description can also be applied to theother constructions of intermediate rolls in the calender. Said intermediate roll 15has been mounted from its ends revolving in bearing housings 151, which have beenmounted on lever arms 152, which have been mounted pivotally on the calenderframe 11 by means of articulated joints 153 fitted in the axial direction of the roll15. The lever arms 152 are provided with support means 154, which are hydrauliccylinders. Thus, said cylinders 154 are attached from one end to the lever arms 152and from the opposite end to the calender frame 11.
• By means of the cylinders 154, a support force is applied to the support constructionsof the roll 15, by means of which force, the loads caused by the weights of theroll 15 and of the related auxiliary equipment, such as the take-out leading roll 155, however, always at least the weight of the auxiliary equipment connected with theroll as added with the weight of the parts placed outside the web, can be compensatedfor and supported in the desired and necessary way. The support can also becarried out so that the loads are supported completely, in which case the weights ofthe roll 15 and of the connected auxiliary equipment have no effect of increasing thenip load. If such complete support is carried into effect in respect of all of theintermediate rolls 15...22, the linear load in each nip N₁ ... N₉ can be made substantiallyequally high.
• Fig. 2A is a schematic illustration of the situation of loading in the set of rolls, inwhich connection each nip N₁...N₉ has an equally high linear load. In this connection,a new term is also introduced in calendering technique, i.e. loading angle α,because this novel mode of loading cannot be illustrated unequivocally in traditionalways. The loading angle α illustrates the distribution of linear load in the set of rollsfrom nip to nip, and in the case of Fig. 2A, i.e. in a case of complete relief, theloading angle α = 90°. By means of said loading angle of 90°, compared withconventional calenders, a significant increase in the calendering potential is obtained.This can be utilized in order to increase the running speed and the productivity.
• The magnitude of the linear load can be regulated fully freely in order to achieve thedesired calendering effect, and, in particular in the case of "full relief", i.e. with aloading angle of α = 90°, the calendering effect can be regulated in the way illustratedin Fig. 2A by way of example. A high linear load and a high calenderingeffect a are employed in order to maximize the running speed of the calender, theproductivity, and the paper quality. A low linear load and a low calendering effecta' are needed under different conditions and in different production stages, such asin matt calendering, in optimizing of quality, in stages of starting up and runningdown, and in situations of web break. By means of a the solution in accordance withthe present invention, a very low calendering effect can be achieved in each nip inthe calender, as is illustrated in Fig. 2A by way of example.
• Fig. 2B illustrates a situation in which, as compared with a calender with a conventionalmode of loading in which the loading angle α is, e.g., 54°, in a mode ofrunning in accordance with the present invention, a loading angle α = 90° isemployed. As is indicated clearly by Fig. 2B, with a mode of running in accordancewith the present invention, a significantly lower level of linear load is needed toproduce similar properties of quality of paper. In this way, it is possible, forexample, to minimize the strain applied to the soft-faced rolls in the calender, suchas polymer-coated rolls, in particular in the lower part of the set of rolls.
• The loads produced by the masses of the intermediate rolls 15...22 in the set of rolls12 and by the masses of the auxiliary devices connected with said rolls can, ifnecessary, also be relieved partially, or so that exclusively the pin loads are relieved,in which case, in respect of the distribution of linear load in the set of rolls, forexample, a situation as shown in Fig. 2C is reached, in which the loading angle αcan be adjusted, e.g., in the range 75°... 80°. Thus, in said situation, the linear loadsare always increasing in the nips when moving towards a lower nip.
• In conventional and traditional supercalenders, the loading angle has, as a rule, beenin the range 45°...55°, and the magnitude of this loading angle has been dependenton the size of the calender, i.e. mainly on the number of rolls. In the method inaccordance with the present invention, the magnitude of the loading angle α can beadjusted quite freely, and by means of this adjustability of the loading angle aconsiderable advantage and a remarkable improvement are achieved over earliersolutions. The loading angle α can be used as an active variable in fine adjustmentof the differences between different faces of the paper. Adjustment of two-sidednesshas a significant effect on the properties of quality of paper, and in this way, bymeans of the present invention, it is possible to produce paper of uniform qualityreel after reel. A corresponding property has not been suggested anywhere elsepreviously.
• The support can, of course, also be accomplished, for example, as a what is called"excessive relief", wherein the loading angle α is larger than 90°. In such a case, it is possible to reach a situation in which a lower nip always has a lower linear loadthan the nip placed above has. Such an embodiment has, however, not been illustratedin the figure.
• In order to establish the significance of the loading angle α and of its adjustabilityas compared with other calendering parameters or variables, quite an extensive testprogram has been carried out with a test machine, and an example of the test resultsis given in Figs. 3A, 3B and 3C, which illustrate the effects of different calenderingparameters with different paper grades. In Fig. 3A the paper grade is SC paper, inFig. 3B the grade is LWC paper, and in Fig. 3C the grade is WFC paper. Theeffects of different factors on the surface properties of paper (gloss,roughness/smoothness) were determined by means of the results, which wereobtained by changing the calendering parameters to a certain extent. The variablesthat were used were running speed, linear load, temperature, and loading angle, asfollows:

  Speed	change in speed 200 metres per minute
  Linear load	change in load 50 kN/m
  Temperature	change in surface temperature of heated roll 15 °C
  Loading angle	change in loading angle from 50° to 90° (50° represents the loading with a traditional mode of supercalendering, and 90° represents an angle which can be obtained with the method in accordance with the present invention)

• As can be seen clearly from Figs. 3A, 3B and 3C, the effect of a change in loadingangle on improvement of the surface properties of paper is higher than with anyother calendering parameter.
• Fig. 1, and so also Figs. 2A, 2B and 2C, illustrate an embodiment in which the setof rolls 12 consisting of the rolls has been installed substantially vertically. Thesolution is, of course, not confined to such an embodiment only, but the set of rollscan be placed in an obliquely vertical position at least to some extent diverging from the vertical position. Of the rolls included in the set of rolls 12, one or several maybe soft-coated polymer rolls and/or paper rolls, fibre rolls or other soft-faced rolls.In the exemplifying embodiment shown in Fig. 1, the upper and the lower roll 13,14are provided with a soft polymer coating, the first, third, sixth, and eighth intermediaterolls 15,17,20, and 22 are hard-faced chilled rolls, and the second, fourth,fifth, and seventh intermediate rolls 16,18,19,21 are soft-coated polymer rolls. Thenumber of the intermediate rolls or the relative sequence and arrangement of thesoft-faced/hard rolls is, however, in no way confined to the exemplifying embodimentof Fig. 1.
• In the method in accordance with the present invention, a situation corresponding toa normal production situation is examined, in which case the set of rolls 12 is closedin the way shown in Fig. 1 and the rolls 13...22 are under load in contact with oneanother. In the way shown in Fig. 1, the automation system 30 included in thearrangement in accordance with the invention has been connected to the supportcylinders 154 to measure and to control the loads of the relief cylinders. In themethod to be examined, in the nips N₁...N₉ in the set of rolls 12, in the runningdirection of the paper web W, a uniform or different, desired distribution of linearload is formed so that in the automation system 30 the deflection lines of theintermediate rolls 15...22 and the corresponding loads of the cylinders 154 ofsupport of the intermediate rolls are computed. The support cylinders 154 and thelever arms 152 are used for supporting the masses of the intermediate rolls 15...22and the masses of the auxiliary devices connected with the intermediate rolls.
• As was already stated with reference to Figs. 2A, 2B and 2C, the distribution oflinear load in the machine direction is regulated by supporting the masses of the rollsand of the connected auxiliary devices completely. Thus, besides the masses of theintermediate rolls, by means of the support cylinders 154 and the lever arms 152,the masses of the auxiliary devices connected with the lever arms of each intermediateroll, such as take-out leading rolls, possible doctors, etc., are also supported.The rigidities and masses of the intermediate rolls 15...22 are not equal from roll toroll. Correcting of the errors in the cross-direction profiles of the deflection lines of the rolls, arising from these differences in rigidity and mass, i.e. regulation of thedeflection lines of the intermediate rolls, is carried out by correcting the loads of thesupport cylinders of the intermediate rolls from their nominal value by means of thenecessary term corresponding to the difference in pressure. The regulation of thedeflection lines of the variable-crown upper roll and lower roll 13, 14 is carried outin the normal way by means of the zone cylinders in the rolls. When the deflectionlines of the variable-crown upper and lower roll 13, 14 are regulated so that they areequal to the deflection lines of the intermediate rolls 15...22, it is possible to givethe set of rolls 12 the desired level of linear load in the machine direction byhydraulically loading either the upper roll or the lower roll. In the case of Fig. 1,this loading can be arranged by means of the lower roll 14, because the loadingcylinders 144 have been connected to act upon the lower roll.
• In the method and the arrangement in accordance with the invention, the necessarycorrection and regulation of the cross-direction profile of paper, i.e. of thicknessand/or glaze, is carried out by means of the zone cylinders in the variable-crownupper and lower roll 13,14. In the intermediate nips, i.e. in the nips N₂...N₈between the intermediate rolls 15...22, correction of the cross-direction profile canbe carried out by means of regulation of the loading of the relief cylinders of theintermediate rolls. The method in accordance with the invention and the relatedcomputing of the distribution of the linear load in the set of rolls 12 can be appliedboth to a traditional mode of running of a multi-nip calender, wherein the paper webW runs through all of the nips N₁...N₉, and to a modified mode of running, whereinthe paper web W is passed through certain nips only. In the method in accordancewith the invention, the automation system includes programs of maintenance of theset of rolls, distributions of linear load, roll parameters, and recipe data bases,which, together with the program of computing of the distribution of linear load,permit computing of the distributions of linear load specifically for each paper grade.Further, for maintaining the changes in the set of rolls in the calender and formonitoring the stock of rolls, there are program routines of their own.
• The distribution of linear load in the set of rolls 12 and the support forces to bepassed to the support cylinders of the intermediate rolls 15...22 are computed eitherin the automation system 30 or in a separate computing unit directly connected withsaid system. The computing model determines the rigidity and the mass distributionof the set of rolls 12 in the calender 10 consisting of chilled rolls and polymer rollsas well as the rigidity of the nips N₁...N₉ between the rolls. Further, in the computing,the locations and masses of the outside masses connected with the set of rollsare determined, the effect of temperature on the modulus of elasticity is taken intoaccount, the effect of the roll diameters on the original modulus of elasticity is takeninto account, a possible additional linear load of the rolls and the separate effects ofthe centres of mass and gravity of the roll ends at the tending side and at the drivingside are taken into account. The data employed in computing are divided into generalcalender-specific, nip-specific, and roll-specific data. Thus, the starting-value datanecessary for the computing are defined in the roll data base 51, in the roll materialdata base 52, in the set-of-rolls mass data base 53, in the data base of geometry ofthe articulated linkage in the calender, i.e. in the set-of-rolls data base 54, as hasbeen illustrated schematically in Fig. 4. In the computing model applied in theinvention, the computing is carried out in two stages so that in the first stage thesupport pressures of the intermediate rolls are optimized and correction coefficientsare obtained for the variable-crown upper and lower rolls. These data are utilized inthe second stage of computing for optimizing the distribution of linear load of theupper roll and the lower roll.
• The way in which the calender in accordance with the invention can be made tooperate in the desired way, i.e. the way in which the forces that support the intermediaterolls are determined, is derived from the procedure in accordance with theinvention, by whose means the ratio of the linear loads applied to the intermediaterolls, of the weight of said rolls, and of the support forces applied to said rolls isadjusted to such a level that a pre-determined state of deflection prevails in the areaof the set of rolls. In the determination of the deflection of each roll, it is alsopossible to include a possible mode of grinding of the roll concerned or of the roll in nip contact with said roll different from cylindrical shape, such as a positive ornegative crown.
• When the basic load and the correction of linear load produced by means of thevariable-crown rolls operating as end rolls are taken into account in the solution ofthe equations of deflection of the intermediate rolls, in every case it is possible toachieve such a state of equilibrium for the set of rolls that the distributions of linearload in the nips in the set of rolls correspond to the desired distribution of linearload.
• The group of equations that has been formed and that illustrates the conduct of theset of rolls can be solved convergently by means of commonly used numeric solutionalgorithms of groups of equations. An example of this is Fig. 5, which illustrates afour-roll supercalender, in which the set of rolls 100 comprises a variable-crownlower roll 111, a variable-crown upper roll 112, and two intermediate rolls 113,114.The nip load in the nips N₁₀₁,N₁₀₂, N₁₀₃ between the rolls is produced substantiallyas the spring force required to produce an elastic compression of the coating on oneof the rolls that form a nip. Since, at each point, the force is proportional to thedifference between the transitions arising in the rolls at the nip, it can be concludeddirectly that at each point the same load is achieved when the difference in transitionat the points is the same, i.e. when the deflection lines of the rolls are of equal shapeand of equal magnitude. Thus, the optimal relief or support of each roll is determinedso that the bending load that remains on each roll mantle produces an equallyhigh deflection on all rolls.
• Since, normally, the deflection forms of rolls are equal (paraboloidal), in theexamination referring to Fig. 5 the deflection of the roll will be describedexclusively by means of the deflection of the centre point of the roll.
• The deflection of a roll as a result of a deflecting linear load produced on the rollmantle can be expressed by means of the formula:δt = k · (qts /(Et · It)) ,from which the load is obtained by means of the deflection:qts = ((Et· It)/k) · δt.
• Herein:

δ_t =

deflection of roll

k =

coefficient depending on mode of loading

q_ts =

linear load that deflects the roll

E_t =

modulus of elasticity of roll

I_t =

inertia of roll

• The sum of the loads that deflect the intermediate rolls in the whole set of rolls:ΔQ = ∑ qts = ∑(((Et · It)/k) · δt)

ΔQ =

change in overall load in the area of the set of rolls

• The load that deflects the roll mantle expressed by means of component loads:qst = Gtv/L + qty - qta + qti

G_tv =

weight of roll mantle

q_ty =

linear load in upper nip of roll

q_ta =

linear load in lower nip of roll

q_ti =

additional linear load arising from other factors in the area ofthe roll mantle

• When it is taken into account that, in an intermediate nip between rolls, the upperand lower nip loads of adjacent rolls are of equal magnitude, the sum of the loadsthat deflect the intermediate rolls in the whole set of rolls is obtained as:ΔQ = ∑ qts = ∑ (Gt/L) + qyy - qaa + ∑ qtl

q_yy =

linear load in the upper nip of the set of rolls

q_aa =

linear load in the lower nip of the set of rolls

• When the deflections of the rolls are denoted equal and when they are substitutedfurther, what is obtained is:δ=δt⇒ΔQ = δ/k · ∑ (Et · It)δ = δt = (ΔQ · k)/∑ (Et · It)
• When this is substituted further in the formula of the load that deflects a roll, whatis obtained is:qts = (Et . It) / ∑ (Et · It) . ΔQ
• Regarding the equilibrium of forces in a roll, the required support force per side issolved:Ftk = ½ · qts · L + Gtp⇒Ftk = ½ · (Et · It) / ∑ (Et · It) · ΔQ · L + Gtp

F_tk=

support force of roll per side

L =

nip length

G_tp =

weight of end parts of roll per side

• The computing of the support forces of the set of rolls in the calender, expressly ofthe whole set of rolls, is based on knowledge of the exact physical properties of the rolls, i.e. the conduct of all the rolls is known when deflecting loads of differentmagnitudes are applied to said rolls. It is the basis of the computing that the bearingsupport forces applied to each roll are determined so that the whole set of rollobtains an equally high calculatory deflection. Thus, by means of regulation of thesupport forces, it is possible to affect the ratio of the upper nip load and the lowernip load at an individual roll so that the sum of these loads, together with the ownmass of the roll, produces the same predetermined deflection in each individual roll.
• The computing can be applied to a set of rolls of any kind whatsoever in a calender,which set of rolls is placed in a substantially vertical position, in which set of rollsthe upper roll is an adjustable-crown roll and the lower roll likewise an adjustable-crownroll, the axial distribution of support forces of said upper and lower roll beingadjustable, and in which set of rolls there are at least two intermediate rolls betweenthe upper roll and the lower roll. Further, it is an important requirement that all therolls in the set of rolls are supported so that their deflection lines are downwardscurved when the nips are closed.
• It is an important characteristic feature of the method, the arrangement, and thecalender in accordance with the invention that, in the computing of the linear loadsin the set of rolls, the physical properties of each intermediate roll that affect thedeflection under load, such as bending rigidity, mass, shape, and material properties,are taken into account.
• It is a further property that the bearing support forces of the intermediate rolls aredetermined by means of computing so that the overall load applied to each intermediateroll subjects each intermediate roll substantially to such a calculatorydeflection that the deflection forms of the contact faces of each roll and of the rollin contact with said roll in a nip substantially correspond to one another.
• The nip forces in a calender are regulated so that the difference between the nipforces of the topmost nip and the lowest nip in the calender is determined to be at the desired level. This means, in fact, the regulation of the loading angle α that wasdescribed in relation to Figs. 2A, 2B and 2C.
• In a summarizing way, it can be stated further that it is an essential feature of theinvention that all the intermediate rolls in the set of rolls are supported to a greaterextent than what is required by the pin forces (all mass outside the web). In such acase, the deflection lines of the rolls are downwards curved and substantiallyparaboloidal. The support forces of each intermediate roll are regulated so that thedeflection of the roll is adapted to the shapes of the other rolls in the set of rolls.Thus, the computing is carried out by means of the deflections. In this way, a groupof equations is obtained in which the basic load between the rolls is determined sothat the deflections of all the rolls are equal. Thus, an equilibrium of forces isproduced in the set of rolls. As the loading angle α it is possible to use any loadingangle whatsoever, and the regulation of the loading angle α is carried out by meansof outside loading members through the lower roll and the upper roll. Thus, in theregulation of the deflection, the variable is the support force with which the roll issupported. The errors produced by the masses of the areas outside the web in thedistribution of linear load (and so also possible other errors in the distribution oflinear load) are corrected by means of the adjustable-crown upper and lower rolls.
• As is shown in Fig. 6, the invention provides a novel possibility of taking care ofthe loading and of the regulation of loading in the set of rolls in a multi-roll calenderby the pair of rolls, which makes the system of regulation simpler and easier tocarry into effect. As was already described earlier, in the present-day supercalenders,as intermediate rolls, as a rule, rolls of two different types are employed,and the rigidities of these two roll types are different. As the intermediate rolls,hard-faced heatable rolls are used, on one hand, and soft-faced rolls are used, on theother hand, which soft-faced rolls can be conventional paper rolls or fibre rolls,which have been formed by fitting disks made of paper or of some other fibrousmaterial onto the roll axle. As soft-faced rolls, to-day, ever increasing use is madeof polymer-faced rolls, in which the roll frame consists of a tubular roll mantle. Therigidities of rolls of the same roll type are substantially equal to one another, but, as was already stated above, the roll types differ from one another essentially in respectof rigidity and, thus, also in respect of the deflection arising from the own mass.
• In a conventional supercalender, the set of rolls comprises a stack of rolls placed ina substantially vertical or obliquely vertical position, wherein the rolls rest one onthe other and the pin loads applied to the bearing housings of the rolls have beenrelieved hydraulically. The loading and profiling of the set of rolls is taken care ofby means of variable-crown upper and lower rolls.
• In the alternative mode of loading shown in Fig. 6, the set of rolls is treated as pairsof rolls 200, which consist of a more rigid roll 202 placed as the lower half in thepair of rolls 200 and of a more flexible roll 201 placed as the upper half. Thus, thedeflection arising from the own mass of this upper roll 201 is higher than thedeflection of the lower roll 202 in the pair. The pairs of rolls 200 in the set of rollsare substantially similar to one another, and they have equal, common deflectionsdepending on the masses and rigidities of the rolls 201,202.
• To the bearing housings of the upper and more flexible roll 201 in the pair of rolls200, for example hydraulically, a force F₂ is applied, by whose means, besidesrelief of the pin loads, the error in the distribution of linear load between the rolls,which error arises from the different rigidities of the rolls 201,202, is compensated.This can be illustrated by means of the formula:2F2 = madd2,wherein

F₂ =

force applied to the bearing housings of upper roll

m_add2 =

mass of the bearing housings and of the auxiliary devicesattached to them as well as the above error arising from differentrigidities of the rolls

• Thus, the upper roll 201 rests with its own weight m₂ (from which the pin loadshave been "cleaned") on the lower roll 202 and applies an even linear load m₂/L to the lower roll, wherein L is the axial length of the nip N between the rolls 201,202.On the other hand, a force F₁ is applied to the bearing housings of the lower roll202 in the pair of rolls 200, by means of which force the masses of both rolls101,102 in the pair of rolls 200 as well as the pin loads of the lower roll 202 aresupported. This can be illustrated by means of the formula:2F1 = m1 + m2 + madd1,wherein

F₁ =

force applied to the bearing housings of the lower roll

m₁ =

mass of lower roll

m₂ =

mass of upper roll

m_add1 =

mass of the bearing housings of the lower roll and of theauxiliary devices attached to them.

• Thus, in an optimal situation, between the separate pairs of rolls 200, no forcesarising from the masses of the rolls are effective at all. In the nip N between therolls 201,202 of the pair of rolls 200, exclusively the linear load arising from themass of the upper roll 201 is effective, for example about 10...20 kN/m. Owing tothe differences between individual rolls, the whole set of rolls must be treated as awhole, and the reliefs of each roll must be optimized so that the cross-directionprofile of linear load of the whole unit is as straight as possible and the linear loadarising from the masses of the rolls is as low as possible. In this way, a set of rollswith almost uniform loading is obtained, which set of rolls is, in the other respects,loaded in the way described above. When, for example, a load of 300 kN/m isconsidered as the load level, in every second nip there is a difference in loading ofabout 5 per cent only, as compared with the preceding or the following nip, i.e.,with existing rolls, a substantially even distribution of load is achieved
• Above, in connection with the description related to Fig. 6, for the sake of simplicity,it has been assumed that the rigidities of the rolls 201,202 in the pair of rolls200 are in a certain ratio to one another and that the rigidities of the rolls belongingto the same type of rolls are equal to one another. However, as was established above in relation to Fig. 5 clearly by means of computing, there would not seem toexist any limitation arising from the mutual ratios of the extents of specific deflectionsof the rolls. Thus any ratio of the rigidities of two rolls whatsoever can becompensated by means of computing so that the magnitudes of the linear loads in thewhole set of rolls can be regulated so that they become substantially equal, with theexception of the deviation caused by the internal nips in calculatory pairs of rolls.
• When conventional upper and lower rolls, for example rolls adjustable in zones, areused, a factor that limits uniform loading is the overall deflection of the intermediaterolls. This limitation could, however, be compensated for so that, if necessary, thelower roll is ground so that its diameter is smaller at the middle than at the ends(negative crown), so that the attainable maximal deflection of the roll adjustable inzones, together with the grinding shape, achieves the maximal possible deflection ofthe set of rolls. In this connection, it should, however, be noticed that, in a set ofrolls of this type, the general direction of deflection of the rolls differs in such a wayfrom the direction of deflection of so-called conventional supercalenders that therolls are in a downwards curved position, in stead of the upward curve formemployed in a conventional supercalender.
• In regulation of loading carried out by the pair of rolls, in the set of rolls in asupercalender, compared with the illustration of Fig. 6, a difference is caused by thereversing nip in the calender, i.e. the nip in which the side of calendering of the webis changed. As a rule, this reversing nip is the middle nip in the supercalender. Thisis illustrated in Figs. 7A, 7B and 7C, in which three alternative modes of loading insaid reversing nip are shown. In said figures, the pairs of rolls as shown in Fig. 6and identical with one another are denoted with the reference numeral 200. In asupercalender, the reversing nip is a nip that is formed between two soft-faced rolls201, and in Figs. 7A, 7B and 7C this reversing nip is denoted with N_e.
• In the solution of Fig. 7A, this has been accomplished so that, in the "pair" of rolls200_e, which is in this case formed by three rolls placed one above the other, thelower roll 202, which is a hard-faced and, for example, heatable roll, has a higher rigidity than the lower rolls in the other pairs of rolls 200. This is because themasses of the two upper rolls 201 rest on the lower roll 202.
• In Fig. 7B, a corresponding solution has been accomplished so that the upper soft-facedroll 201_e1 in the reversing nip N_e is arranged as a variable-crown roll. In sucha solution, the deflection of said roll 201_e1 is corrected by means of the crownvariation means fitted in the interior of the roll, and the mass of the roll does notload the pair of rolls 200_e1 placed underneath by means of its weight.
• In Fig. 7C, a corresponding solution has been accomplished so that the upper soft-facedroll 201_e2 in the reversing nip N_e has been arranged as a roll with such arigidity that its deflection is the same as the deflection of the whole pair of rolls200,200e₂. In such a case, said roll in the reversing nip does not cause any problemin the regulation of the loading.
• With reference to Fig. 8, in the computing, in accordance with the invention, firstthe initial values of the rolls are defined, and on this basis the mathematical modelcorresponding to the set of rolls is formed. The mathematical model is formed incompliance with the number of rolls included in the set of rolls. The optimizationcomputing formed for the set of rolls uses these data as the starting data. In theoptimization computing that is to be carried out, the nip errors of the intermediaterolls are minimized, which errors have been defined as deviations from the nominalform. The resilience occurring between each nip and arising from the paper andfrom the coatings is illustrated by a base constant, which is computed across the niplength. The effects of the forces to be optimized on the linear load are determinedin a response data base, in which the unit response of the element of the nip of eachintermediate roll is indicated in a desired number of examination points. The effectsof invariable forces on the linear load are determined in a separate invariable-forcedata base, which takes into account divided masses, point masses, and nips withinvariable load. Further, for the computing, the effects of the forces to be optimizedon the restrictions and the effects of backup forces on the tension restrictions aredetermined. Thus, the assignment of optimization becomes a mathematical problem, in which the variables are limited and determined by groups of equations. As a resultof the computing, optimal relief forces for intermediate rolls, optimal profiles oflinear load and deflections of rolls are obtained.
• After the computing operation, the optimized support forces of the intermediate rollsin the set of rolls of the calender are transferred to the support cylinders of intermediaterolls, as is illustrated, for example, in Fig. 1. The optimized support forcesof intermediate rolls are also transferred to the program of computing of the zonepressures of the variable-crown upper and lower rolls. The deflection values of theintermediate rolls in the set of rolls are used for controlling and regulation of thevariable-crown upper and lower rolls. From the deflection values of the intermediaterolls, by means of a separate computing program, the zone pressure corrections ofthe upper and the lower roll are determined, which corrections are, in each particularcase, added to, or reduced from, each actual value of zone pressure. Thedistribution of linear load in the set of rolls is controlled in the method in accordancewith the invention so that, by means of the user interface of the automation system,first the desired form of the distribution of linear load is determined. After this, theautomation system and the included computing programs compute the above setvalues for the support pressures of the intermediate rolls and for the zone pressuresof the variable-crown upper and lower rolls. The method in accordance with theinvention also takes into account situations of change in the set of rolls arising fromchange of roll or from a new mode of running as well as any changes arising fromsaid situations of change in the set-of-rolls data base and in the parameter data basesand in the computing. Likewise, in its roll and material data bases, the methodcovers and takes into account situations in which the diameters and/or materialproperties of chilled rolls and/or polymer rolls are changed.
• As regards the process conditions of calendering, it can be stated generally that theyare determined by the capacities of the components that are used as rolls, as is alsoordinary in calender technology. Further, restrictive factors in the process includethe desired properties of paper, such as bulk (stiffness), smoothness/roughness, andgloss, in particular gloss of printing paper. As examples of process conditions, the US Patents 4,749,445 and 4,624,744 of S.D. Warre can be stated. A possible rangeof surface temperature of a heatable, so-called thermo roll is T_s = 60 °C ... 250°C, depending on the running speed so that the surface temperature is lower at lowrunning speeds and higher at high running speeds, because the time of effect of thenip is shorter and, thus, the transfer of heat from the thermo roll to the web face islower. The range of variation of linear load can be 20 kN/m ... 550 kN/m or evenhigher, again depending on the running speed and on the properties of the variable-crownupper and lower rolls that produce the linear load in the supercalender.
• Above, the invention has been described by way of example with reference to thefigures in the accompanying drawing. The invention is, however, not confined to theexemplifying embodiments shown in the figures only, but different embodiments ofthe invention may show variation within the scope of the inventive idea defined inthe accompanying patent claims.
• The invention concerns a method and an arrangement forcomputing and regulation of the distribution of linear loadin a multi-nip calender. The material web (W) is passedthrough the nips (N1...N9) in the set of rolls (12), whichset of rolls comprises a variable-crown upper roll (13), avariable-crown lower roll (14) and intermediate rolls(15...22) fitted between the upper and lower rolls (13,14). All the rolls in the set of rolls are supported sothat, when the nips (N1...N9) are closed, the bending linesof the rolls are curved downwards. In the computing andregulation of linear loads, the physical propertiesaffecting the bending of each intermediate roll (15...22)under load, such as bending rigidity, mass, shape, andmaterial properties, are taken into account. The ratio ofthe linear loads applied to the intermediate rolls(15...22), the own weight of the rolls, and of the supportforces applied to the rolls is regulated so that the set ofrolls is in a state of equilibrium and in a predeterminedstate of deflection. The invention also concerns a multi-nipcalender that carries out the method.

Claims (10)

1. A method of controlling a multi-nip calender includingcomputing and regulation of the distribution of linearload, wherein in the multi-nip calender the material web(W) to be calendered is passed through the nips (N₁...N₉)in a set of rolls (12), which set of rolls is formed by avariable-crown upper roll (13), a variable-crown lower roll(14) and by at least two intermediate rolls (15...22)provided with support cylinders (154) and fitted betweenthe upper and lower rolls (13, 14), in which connection allthe rolls in the set of rolls are supported so that, whenthe nips (N₁...N₉) are closed, the bending lines of therolls are curved downwards,characterized in that in thecomputing and regulation of linear loads, the physicalproperties affecting the bending of each intermediate roll(15...22) under load, such as bending rigidity, mass,shape, and material properties, are taken into account, andthe ratio of the linear loads applied to the intermediaterolls (15...22), of the own weight of the rolls, and of thesupport forces applied to the rolls is regulated so thatthe set of rolls is in a state of equilibrium and in apredetermined state of deflection,
      comprising computerized modeling of all essentialelements of the multi-nip calender including determiningthe physical properties of all rolls and selecting the typeand position of each roll in the multi-nip calender,
      determining of regulation parameters based on thecomputerized modeling,
      regulating of the multi-nip calender assembled withthe types and positions of rolls used in the computerizedmodeling based on the computerized modeling.
2. A method as claimed in claim 1,characterized in that,in the method, in the set of rolls (12) in the calender, intermediate rolls (15...22) are employed whose deflectionproperties are different from roll to roll.
3. A method as claimed in claim 1 or 2,characterized inthat, in the computing, the set of rolls (12) is treated asa single unit.
4. A method as claimed in claim 1 or 2,characterized inthat the computing is carried out by a pair of rolls (200).
5. A method as claimed in any of the preceding claims,characterized in that, in the method, the intermediaterolls (15...22) in the set of rolls (12) are supported onthe frame (11) of the calender so that the rolls are freelymoving.
6. An arrangement for computing and regulation of thedistribution of linear load in a multi-nip calender meantfor calendering of paper or board, which calender comprisesa set of rolls (12) which is mounted on the frame of thecalender and which set of rolls includes a variable-crownupper roll (13), a variable-crown lower roll (14) as wellas one or several intermediate rolls (15...22) fittedbetween the upper roll and the lower roll, in whichconnection the means of suspension of the intermediaterolls (15...22) are provided with support cylinders (154),and all the rolls in the set of rolls are supported sothat, when the nips (N₁...N₉) are closed, the bending linesof the rolls are curved downwards,characterized in thatthe arrangement includes an automation system (30) and acomputing unit (40), which have been fitted, in thecomputing and regulation of linear loads, to take intoaccount the physical properties affecting the bending ofeach intermediate roll (15...22) under load, such asbending rigidity, mass, shape, and material properties, andto regulate the ratio of the linear loads applied to theintermediate rolls (15...22), of the own weight of the rolls, and of the support forces applied to the rolls sothat the set of rolls is in a state of equilibrium and in apredetermined state of deflection,
      wherein the computing unit (40) computerized modelsall essential elements of the multi-nip calender includingthe determination of the physical properties of all rolls,wherein the type and position of each roll in the multi-nipcalender is selected,
      the automation system (30) regulates the multi-nipcalender based on the computerized modeling assembled withthe types and positions of rolls used in the computerizedmodeling.
7. An arrangement as claimed in claim 6,characterized inthat the arrangement has been arranged to regulate the setof rolls (12) in the calender, in which the intermediaterolls (15...22) have deflection properties different fromroll to roll.
8. An arrangement as claimed in claim 6 or 7,characterized in that the arrangement has been fitted, incomputing, to treat the set of rolls (12) as a single unit.
9. An arrangement as claimed in claim 6 or 7,characterized in that the arrangement carries out thecomputing by a pair of rolls (200).
10. A multi-nip calender controlled by the method asclaimed in any of the claims 1 to 5.

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