EP1275776A1 - Calender and method of arranging rolls in a calender stack - Google Patents

Abstract

A calender is specified with a roll stack (1) which has two end rolls (2, 3) in a press plane (7) and in between several middle rolls (4-6), at least one (5) of which has an elastic surface (9) having. Furthermore, a method for arranging rolls in a roll stack of a calender is specified, which has two end rolls in a press plane and between them several middle rolls, at least one of which has an elastic surface, and which runs in operation at a nominal speed. One would like to increase the service life of the elastic roller. For this purpose, in the calender (1) at least one roller (5) with an elastic surface (9) has an offset (x) with respect to the press plane (7), the size of which is selected as a function of the wavelength of a critical natural frequency within the roller stack. The rollers are arranged in such a way that the natural vibrations of the calender (1) are determined at the nominal speed, a critical natural vibration is selected from the natural vibrations, a wavelength belonging to the natural vibration is determined, the integral multiple of which corresponds to the circumference of the roller (5), and the Roller (5) offset so that a path length difference on the surface of the roller (5) between two nips (11, 12) of a quarter wavelength arises. <IMAGE>

Description

The invention relates to a calender with a roll stack, of two end rollers and between them has several center rollers, of which at least one has an elastic surface. The invention further relates to a method for arranging of rolls in a roll stack of a calender, of the two end rolls in a press plane and in between has several center rollers, at least one of which has an elastic surface, and that in operation runs at a nominal speed.

The invention is based on a calender described, for satinizing paper or cardboard webs is used. But it is in the same way also applicable to other material webs where similar problems occur.

When satinizing a paper web, the paper web becomes passed through the calender and into nips between a hard and a soft roller, i.e. a roller with elastic surface, are formed, with increased Pressure and possibly also at elevated temperature applied. For newer calenders, for example the "Janus calenders", rollers are used, which are covered with a plastic covering. One can now observe that in many cases there is a certain Operating time for horizontal stripes on the paper web comes. As soon as these horizontal stripes become visible the paper web unusable and forms committee. The reasons this so-called barring formation are currently not yet fully clarified. But one assumes that it concerns the effects of a vibration phenomenon is. Vibrations are in a calender though practically inevitable.

Barring phenomena in themselves are also earlier occurred and that in smoothing, i.e. for calenders, which were only equipped with hard rollers. Here, however, it is assumed that the causes of the Barring formation was to be found in the paper web, i.e. the periodic occurrence of changes in thickness for example a slightly pulsating one Headbox have been caused.

An attempt has been made to do this in the calender To avoid barring formation either by: a guide roller at varying distances from the roller stack arranges or one or more rollers laterally offset from the press level.

Barring formation on soft rollers, in particular on plastic rollers, but it is a different one Appearance. It can be observed here that the elastic surface layer even in a relatively short Time reshaped. If a barring phenomenon occurs, the roller, which has the barring formation, must be removed and ground or turned off. The The service life of such a roller is therefore limited.

When the barring is formed, the soft roller is changed on their elastic surface. It is still did not finally clarify exactly how this change looks. The following options are currently accepted: The roller gets a ripple on the surface, i.e. a mountain and valley structure, the roller becomes polygonal or the roller gets alternately in the circumferential direction Zones of different surface quality, for example different roughness. Independently the concrete nature of the change is evident the periodic barring formation, running in the axial direction Strip on the circumference of the roller. Appropriate Stripes then appear on the paper web, at the latest from the time the strips become visible the paper web is to be regarded as a committee.

The object of the invention is the service life to increase such a roller.

This task is performed on a calender of the type mentioned at the beginning Kind solved in that at least one roller with an elastic surface opposite the press plane has an offset, the size of which is selected depending on on the wavelength of a critical natural frequency inside the roll stack.

You build the calender from the start so that at least a soft roller with one without offset Barring formation could occur opposite the press level is transferred. To do this, first determine a critical natural frequency of the calender. A stack of rollers, which is made up of several rollers has one Variety of natural frequencies. Here are not the Natural frequencies of the individual rollers, such as Bending natural frequencies, meant, but the natural vibration forms, resulting from the vibrating roller masses on the spring and damper systems of the intermediate Plastic coverings of the "soft" rollers result. A running calender generates excitation forces Frequencies arise from multiples of the roller speeds put together. These excitation forces can Inhomogeneities, anisotropies or geometry errors (Out of roundness). Fluctuations in paper thickness can also occur the paper web running through the calender stimulate the roll stack. One in the calender incoming paper web is still before the smoothing process very rough. In addition, a paper web is never free of basis weight or thickness fluctuations. One analyzes these fluctuations with the help of an FFT analysis on their Frequencies, so you usually put a broadband Noise fixed, in which all frequencies are contained are. If one of these excitation frequencies hits one Natural frequency, so the vibration system of the Calender with increased vibrations. by virtue of the multitude of possible pathogens and the multitude of the possible forms of natural vibration constructively avoid these resonance points. In the As a rule, the vibration system is so strongly damped and the excitation forces are so small that the resulting ones Swinging movements not immediately disturbing are. Emboss over a more or less long period however, these oscillating movements in the plastic coverings of the elastic rollers.

Usually the closest to the natural frequency integer multiples of the roll rotation frequency embossed as a pattern on the rollers. This is done a feedback of the vibration. The vibration rashes then increase exponentially. You express yourself on the one hand in an increased sound level (up to more than 120 dB (A)) and on the other hand in periodic fluctuations in thickness the continuous paper web. In practice different periods are observed in who are experiencing these feedback phenomena express in barrings, train. Most of them pass Days or weeks until this phenomenon grew so much is that it disrupts the production process.

Not all of these natural frequencies are critical. Frequencies that are relatively low affect usually not disturbing the rollers. frequencies that are relatively high may generate Barrings on the paper web. These horizontal stripes lie but then so close together that they are basically are indistinguishable. Let the natural vibrations calculated using known numerical methods, for example with methods using finite elements work. Programs for this are commercially available. A program that calculates the natural vibrations is available under the name "Ansys".

The offset preferably causes a path length difference on the surface of the roller between two nips by a quarter wavelength. This approach has several advantages. First, the offset is relative small. It is usually of the order of magnitude 10 mm, often also below, so that the Offset no significant change in geometry of the roll stack results. So you can still assume that the press forces also in the press plane Act. But above all, this configuration the advantage that there is a barring formation in the critical Natural frequency does not result or at least in time is very much delayed. Here one starts from following consideration. Over time can only add up the wavelengths on one roll circumference, their integer multiple equal to the circumference of the roller is. All other wavelengths are deleted with the Time out again. So there are integer multiples of the roll rotational frequencies that are close to a Natural frequency lie, possible frequencies, which turn out to be Train barring. The number of wavelengths being imaged depends not only on the proximity to Natural frequency, but also on the shape of the vibration. The mode of vibration is decisive for whether a even integer multiple or an odd represents an integer multiple of the roll rotation frequency. With an even multiple, the elastic Roll on both sides of each shaft, so to speak from charged. With an odd multiple there is a burden on one side and a relief opposite on the other side. Now if you have one Path length difference on the surface of the roller caused by a quarter wavelength, there is one Phase shift of the waves by π / 2. In this case couple the two nips on which the soft roller participates is no longer directly with each other. A The individual nips can only feed back to themselves disturbed by a change in the roller speed over time become.

The offset is preferably one eighth of a wavelength. You can see the path length difference on the Surface of the roller by a quarter wavelength thereby generate an eighth wavelength for each nip added (on one half of the roller) or removed (on the other half of the roller). The offset can thus be kept relatively small overall.

The roller preferably has an adjusting device with the help of which the offset starting from a preset offset, which depends on the wavelength, is adjustable. Such an adjustment device is particularly beneficial for calenders that have multiple have critical natural frequencies. In this case you can with the originally set roll offset prevent the formation of barrings or delay based on a natural frequency. Therefore but then barrings may form that are based on a different natural frequency. If you now has the possibility to change the roll offset, then you can choose between several positions the rollers switch back and forth to all critical Natural frequencies to the formation of barrings to disturb.

Preferably, the calender has a predetermined Frequency range only a single critical natural frequency on. This can be done with constructive measures achieve, for example by selecting suitable ones Diameter combinations of the rolls. If in that critical area only a single natural frequency occurs then the fight against the barrings by the Realizing offset relatively safely.

The task is in a method of the aforementioned Kind solved in that the natural vibrations of the calender at the nominal speed, a critical natural vibration from the natural vibrations selects one belonging to the natural vibration Wavelength determined, its integer multiple corresponds to the circumference of the roller, and the roller so offset that a path length difference on the surface the roller between two nips of a quarter wavelength arises.

As stated above in connection with the calender, results from a path length difference on the surface the roller of a quarter wavelength one Phase shift when the roller is loaded in the two nips of π / 2. In this case they couple two nips no longer directly with each other. You go assume that with otherwise equal excitement Vibration paths in the individual nips only with half the intensity memorize if there is no feedback between the two In theory, nips would result in each other the service life can be doubled.

It is preferable to choose an odd multiple. Usually a wavelength is exactly one Natural frequency belongs, not an integral part the circumference of the roller. So there are in the neighborhood this "exact" wavelength two wavelengths, that could be critical. One wavelength results if it has an even integer is multiplied, the roll circumference. The other wavelength results if it is with odd whole Number is multiplied, the roll circumference. You choose that is, the wavelength with an odd Number multiplied gives the circumference of the roller. It has It has been shown that a longer service life of the elastic rollers achieved.

The natural frequency is preferably divided by the Rotational frequency of the roller and receives a theoretical Barring number as quotient, whereby the multiple is the next is an integer of the theoretical Barring number. This is a relatively simple multiple-fold approach to investigate. It has been found that achieved good results with this multiple.

The roller is preferably offset by one eighth of a wavelength. As explained above in connection with the calender, is this enough to put both nips together a path length difference of a quarter wavelength to effect.

The set offset is preferably changed. With this you also have the calender in operation, if necessary during breaks, a possibility of correction.

Fig. 1

eine schematische Darstellung eines Kalanders,

Fig. 2

verschiedene Möglichkeiten zum Einstellen eines Versatzes einer Walze und

Fig. 3

eine schematische Darstellung zur Erläuterung der Ausbildung eines Barring-Musters.

The invention is described below with reference to a preferred embodiment in connection with the drawing. Show here:

Fig. 1

a schematic representation of a calender,

Fig. 2

different ways of adjusting an offset of a roller and

Fig. 3

a schematic representation for explaining the formation of a barring pattern.

Fig. 1 shows schematically a calender 1 with two end rolls 2, 3, which are designed as deflection rollers and three center rollers 4-6, which together form one Form the roll stack. The roll stack has a roll plane 7, in which the axes of all rollers 2 - 6 lie when the rollers 2 - 6 are arranged exactly one above the other are. In this roller plane 7 lies for the For the purpose of the following description also the press direction, i.e. the direction in which the reels 2-6 pressed against each other.

Further details of the calender are only schematic shown as a drive 8, or omitted entirely, like means for heating individual rollers. The two End rollers 2, 3 and the middle roller 5 have but an elastic covering 9, which is exaggerated is shown in bold.

The rollers 2 - 6 form during operation of the calender in known manner nips 10-13, through which one to be treated Material web is guided. All nips are here formed as so-called soft nips because they are from a hard and limited by a soft roller.

The middle roller 5 is offset by a distance X. The path X accordingly forms an offset of the Roller. This offset is calculated beforehand. The for this necessary considerations should first be based on Fig. 3 will be explained.

3 shows the roller 5, the one above it Roller 4 and the roller 6 below. With exaggerated amplitudes there are different reference ripples shown, namely a ripple, in which seven shafts run around the circumference of the roller 5, one with eight waves and one with nine waves. The numbers n = 7, 8, 9 were for the sake of clarity selected. With real rollers, the Adjust the circumference of the roller accordingly more shafts, for example in the order of 30 to 50. At the number of waves around the circumference of the roller 5 in the first approximation, one can assume that with a small offset movement of the roller 5 opposite the roller plane 7, which is smaller than one Wavelength, the curvature of the roller 5 does not matter plays.

The offset X ensures that the distance between the two nips 11, 12 on one side a quarter wavelength λ / 4 enlarged and on the other Side reduced by this quarter wavelength λ / 4 becomes. All that is required is an offset X, which corresponds to X = λ / 8, because the desired one Path length difference between the two nips 11, 12 results.

Due to the static roller offset, which is a path extension between the two nips 11, 12 of a quarter Wavelength λ / 4 causes, it can be assumed that yourself with the same excitement by coupling with the two neighboring rollers the disturbances separately impress with half intensity, so that theoretically the service life can be doubled.

The procedure for calculating the offset should will now be explained using an example. The calender should have a nominal speed of 1280 m / min, i.e. all rollers are designed to run at a peripheral speed of 1,280 m / min. Here it is assumed that the roller 4 has a diameter of 870 mm, the Roller 5 has a diameter of 874 mm and roller 6 has a diameter of 878 mm. The roller circumference is calculated corresponding to 2733.1855 mm, 2745.7520 mm and 2758.3184 mm.

With a finite element method it was previously determined that a system natural frequency fe of 277.3120 Hz exists with the system natural frequency form asymmetrical to the roller 5.

From the roll sizes mentioned above and the intended Production speed, i.e. the nominal speed, a roll rotation frequency is calculated fw of 7.8053 Hz, 7.7696 Hz and 7.7342 Hz for the rollers 4, 5, 6. This gives the quotient fe / fw a theoretical Barring number of 35.5287, 35.6920 and 35.8554 for rollers 4, 5, 6. As the closest Barring number becomes the closest integer odd number accepted. This is the number 35. Without the offset it would one assume that 5 on the roller Barring pattern forms with a wavelength that the Circumference (2745.752 mm) divided by 35 corresponds to a wavelength of 78.4501 mm.

If you now the roller 5 by the roller offset X = 78.4501 mm / 8 = 9.8063 mm offset, then with one very likely to assume that there is no barring formation at this wavelength or shows very late. The service life of the elastic Roll 5 is increased dramatically by the offset X.

1 is designed so that it only a system natural frequency fe of 277.3120 Hz in the critical Area. The critical area here is a frequency range in which barrings can occur. Frequencies above or below this range are in any case not critical for the formation of barring.

If a calender 1 has several critical frequencies, measures can be taken in advance, to make an adjustment even after the roll stack has been formed of the offset. Examples of this are given in Fig. 2.

Fig. 2 now shows different ways to To cause roll misalignment. The explanation is given in all cases using the example of the center roller 5, which in one Bearing housing 30 is mounted, which is on the front End of a lever 31 is located.

In the exemplary embodiment according to FIG. 2a, the lever is 31 stored with a bearing point 32 in an eccentric sleeve 33. If the eccentric bushing 33 is rotated, then the position of the roller 5 changes in horizontal Direction.

In the embodiment according to FIG. 2b, the lever is 31 stored in a sliding block 34, which is in a housing 35 by a linear drive 36, which is only schematic is shown, can be moved in the housing 35. The linear drive can be used, for example, as a threaded spindle will be realized. Also with a threaded spindle relatively precise adjustment movements are possible.

In the exemplary embodiment according to FIG. 2 c, the lever 31 adjustable in length, which is indicated by a double arrow 37 is shown. The lever 31 can for example have a telescopic or a prismatic guide. The drive of the two movable against each other Parts of the lever can also be threaded (not shown).

2d is the bearing housing 30 connected to the lever 31 via a swivel joint 38. The swivel 38 is at the lower end of a mounting plate 39 arranged, which in turn on the lever 31 is attached. An attachment is at the top of course also possible. A schematically represented Tilt drive 40 is provided around the bearing housing 30 compared to the lever 31 by a defined amount to tip.

The adjustment path is designed so that it becomes one Offset X leads from the press level 7, which in turn sufficient to form a barring pattern to interfere on the surface of the elastic roller or eliminate again. To make a barring pattern again eliminate, it may be appropriate to offset X = λ / 4 to choose, i.e. a path length difference of λ / 2 to cause the surface of the roller 5, wherein λ the Is the wavelength of the newly occurring barring pattern.

Claims (10)

Calender with a roll stack which has two end rolls in a press plane and in between several middle rolls, at least one of which has an elastic surface, characterized in that at least one roll (5) with an elastic surface (9) is offset from the press plane (7) (x), the size of which is selected as a function of the wavelength of a critical natural frequency within the roll stack (1). Calender according to claim 1, characterized in that the offset (x) causes a path length difference on the surface of the roller (5) between two nips (11, 12) by a quarter wavelength. Calender according to claim 1 or 2, characterized in that the offset (x) is an eighth wavelength. Calender according to one of claims 1 to 3, characterized in that the roller (5) has an adjusting device (33, 36, 37, 40) with the aid of which the offset is based on a preset offset (x) which depends on the wavelength , is adjustable. Calender according to one of claims 1 to 4, characterized in that it has only a single critical natural frequency in a predetermined frequency range. Method for arranging rolls in a roll stack of a calender, which has two end rolls in a press plane and in between several middle rolls, at least one of which has an elastic surface, and which runs in operation at a nominal speed, characterized in that the natural vibrations of the calender determined at the nominal speed, a critical natural vibration is selected from the natural vibrations, a wavelength belonging to the natural vibration is determined, the integral multiple of which corresponds to the circumference of the roller, and the roller is offset such that a path length difference on the surface of the roller between two nips of a quarter Wavelength arises. Method according to Claim 6, characterized in that an odd multiple is chosen. Method according to Claim 6 or 7, characterized in that the natural frequency is divided by the rotational frequency of the roller and a theoretical Barring number is obtained as the quotient, the multiple being the next whole number to the theoretical Barring number. Method according to one of claims 6 to 8, characterized in that the roller is offset by an eighth wavelength. Method according to one of claims 6 to 9, characterized in that the set offset is changed.

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