EP1275775B1 - Process for operating a calender - Google Patents

Description

The invention relates to a method of operating a calender with a roll stack comprising two end rolls and, between them, a plurality of center rolls abutting one another in a press direction, at least one roll being formed by a soft roll having an elastic surface.

Such calenders are used in particular for calendering paper or board webs. The invention will be described below with reference to the treatment of a paper web. However, it is equally applicable to other webs where similar problems occur.

When satinizing a paper web, the paper web is passed through the calender and into nips between a hard and a soft roll, ie a roll are formed with elastic surface, subjected to elevated pressure and optionally also with elevated temperature. In calenders newer design, for example, the "Janus calenders" rollers are used, which are covered with a plastic covering. It can now be observed that in many cases, after a certain period of operation, horizontal stripes occur on the paper web. Once these streaks become visible, the paper web is useless and forms reject. The causes of this so-called barring formation are currently not fully understood. But it is believed that these are effects of a vibrational phenomenon. Vibrations are virtually unavoidable in a calender.

In barring formation, the soft roll is changed on its elastic surface. It has not yet been clarified how this change looks exactly. Currently, the following possibilities are assumed: The roller gets a waviness at the surface, i. a mountain and valley structure, the roller becomes vielekkig or the roller gets in the circumferential direction alternately zones of different surface quality, for example, different roughness. Regardless of the specific type of change, the barring formation shows periodic, axially extending strips on the circumference of the roll. Corresponding strips then show up on the paper web, wherein at the latest from the becoming visible of the strip, the paper web is to be considered as a committee.

If a barring phenomenon occurs, the roller that causes the barring formation must be removed and ground or turned off. The life of such a soft roll is therefore limited.

The appearance of the barring is also known from calenders, that is calenders, which are formed exclusively of hard rollers. However, the formation of the bar ring on the paper web took much longer. It is believed that the barring formation is due to other causes, in particular disturbances in the paper web.

In "Calender Vibration - A Simulation Study and a Cure", Tappi Journal, Vol. 52, no. 7, July 1969, pages 1356 to 1361, it is proposed to design a guide roller so that its position is variable relative to the roll stack. This is intended to simulate a diameter change of a roll.

"Calender barring on paper machines - practical conclusions and recommendations", Tappi Journal, Vol. 8, August 1975, pages 147-151, starts from a similar solution, alternatively proposing to offset the rolls between 20 and 60 mm relative to the roll stack. However, this offset has the disadvantage that it changes the geometry of the roll stack relatively strong, which can adversely affect the Strekkenlasten and the line load distributions in the individual nips.

In EP 0 949 378 A1, vibrations which occur over a relatively long period of time are cited as the cause of barring. Stationary operation is by a change in the drive torque distribution avoided at least two driven shafts or by different throughput speeds. As a result, Barringerscheinungen be reduced and increases the life of soft rolls.

In US 3 044 392 the barring is attributed to horizontal oscillations (transverse to the press direction) of the superposed rolls. These vibrations can be avoided by neutralizing the horizontal forces introduced by guiding the paper web. This is achieved by staggering the axes of the rolls horizontally. If several rollers are driven, the oscillation tendency can be further reduced by introducing additional drive torques.

With the use of plastic-based rollers, a new type of barring has emerged in which patterns are imprinted on the surface of the soft rolls in a relatively short time.

The invention has for its object to increase the service life of a soft roll.

This object is achieved in a method of the type mentioned above in that one determines a wavelength of the barring pattern on occurrence of a barring mousse on the circumference of the soft roll and makes a roll offset of a soft roll transversely to the press direction in response to this wavelength.

As a rule, a barring pattern on the surface of a roller can already be established before this barring pattern impresses itself in the form of transverse strips in the paper web. So if you take measures in good time to disrupt the stronger character of the barring pattern, then you can increase the service life of the roller. This is based on the following considerations:

A stack of rolls, which is formed from a plurality of rolls, has a multiplicity of natural frequencies. This does not mean the natural frequencies of the individual rolls per se, such as bending natural frequencies, but the natural vibration modes which result from the oscillating roll masses on the spring and damper systems of the interposed plastic linings of the "soft" rolls. A running calender generates exciter forces whose frequencies are composed of multiples of the rolling speeds. These excitation forces can be due to inhomogeneities, anisotropies or geometrical errors (roundness). Also, paper thickness variations of the calender passing through the paper web can stimulate the roll stack. A paper web entering the calender is in front of the smoothing process still very rough. In addition, a paper web is never free of basis weight or thickness variations. If one analyzes these fluctuations with the aid of an FFT analysis on their frequencies, one usually detects a broadband noise in which all frequencies are contained. If one of these excitation frequencies hits a natural frequency, then the vibration system of the calender responds with increased oscillations. Due to the large number of possible pathogens and the large number of possible natural modes of vibration, these resonance points can not be avoided constructively. In general, the vibration system is so strongly damped and the excitation forces are so small that the resulting oscillatory movements are not directly disturbing. Over a more or less long period of time, however, these oscillatory movements are reflected in the plastic coverings of the elastic rollers.

Usually, the closest to the natural frequency integer multiples of the rolling rotational frequency are impressed as a pattern on the rollers. This results in a feedback of the oscillation. The oscillations then increase exponentially. They express themselves on the one hand in an increased sound level (up to more than 120 dB (A)) and on the other hand in periodic thickness variations of the continuous paper web. In practice, different periods of time are observed in which these feedback phenomena manifest themselves in barrings. Most days or weeks pass before this phenomenon has grown so strongly that it disturbs the production process.

If one analyzes the barring patterns on the surface of the roll, one finds that this is a wave pattern in which the wavelength is an integer fraction of the roll circumference. It is now possible to evaluate this information and move the roller across the press direction by an amount into which this wavelength information flows. Since one uses the wavelength information selectively, the offset of the roller can then be kept constant again for some time. An offset is therefore required only at longer intervals. It is now possible to operate the calender until a barring pattern on the surface has developed so strongly that disturbances are to be feared in the foreseeable future. The occurrence of such a barring pattern can be determined, for example, by vibration or vibration measurements on the calender or even on each individual roller. If you then makes the roller offset at a time that is just before a "critical" time at which the roller should actually be replaced, then you can achieve that the barring pattern re-embossed or even completely disappears, so that the The life of the roller almost double by a single offset operation, but can usually extend by at least 30%.

Preferably, one takes a roll offset, which is smaller than the wavelength. This procedure has several advantages. On the one hand, the displacement movement is limited, ie disturbances that result from removal of the roll from the rolling plane of the calender are kept small. On the other hand, the structural or handling measures that must be taken for the roller offset, limited. Essentially could theoretically increase the roll offset by integer multiples of the wavelengths. The restriction to the smallest possible distance but brings the above advantages.

Preferably, a roll offset is made on the roll at which the barring pattern occurs. This eliminates the disturbance immediately at the place of origin. Especially with center rolls you can then use two nips for the elimination of the barring pattern.

Preferably, it is provided that a roller displacement is performed, which causes on the surface of the roller a wavelength difference of half a length in a first case and a quarter wavelength in a second case, in the first case, a barring pattern on the surface of the roller occurred, which is to be eliminated, and in the second case, an elimination is not required. In this approach, it is assumed that the barring pattern has formed on the surface of the roll by a vibration in which the adjacent roll or the adjacent rolls always exert a strong load on the roll at certain points on the circumference of the roll and at others Points that are offset by half a wavelength to a less intense load. Center rollers, in which the formation of such barring pattern is observed particularly frequently, are also burdened by the interaction of both neighboring rollers. If you now change the path on the roll surface between two nips by half a wavelength, ie extended or shortened by half a wavelength, then, with otherwise unchanged conditions, the "wave crests" will always be at the place of highest load and the "wave troughs" will be at the place of lowest load. The barring pattern will re-emboss relatively quickly in this way. It is indeed accepted that forms a new Barring pattern. However, the time required for the "old" barring pattern to disappear and the time spent to form the "new" barring pattern increases the life of the roll. If elimination of the barring pattern is not (yet) required, then one can also provide for changing the path on the roll surface between two nips by only a quarter wavelength. If previously barring patterns have formed that have not yet exceeded a tolerable level, eliminating barring patterns is often unnecessary.

Preferably, roll offset is effected by half the path length difference when the roll is a center roll. The path length difference is basically the transit time difference between two nips. If one adds each of these two nips half of the path length difference, then it is sufficient to offset the roller altogether by half of this path length difference. This offset movements of the roller are kept extremely small. It is necessary to make the offset relatively accurately. However, this is easily possible, as will be described below.

Preferably, one makes a path length difference of a quarter wavelength when replacing a roller against a structurally identical replacement roller. With an identical replacement roller, it can be assumed that the same vibration phenomena occur. Since a spare roll but no barring pattern has emerged, a removal is not required. One can therefore take a "prevention" and by offsetting the roller against a situation in which barring patterns have formed, ensure that the barring pattern just does not form. Although this procedure does not ensure that another barring pattern is formed. The likelihood is reduced. If another barring pattern is formed, this can be remedied again with the measures described above. The life of the roller is increased in any case.

Preferably, the roll offset is achieved by changing the length of a lever on which the roll is mounted. This is a relatively simple measure to relocate the roller. The roller may then again remain firmly in position for some time, which has been determined by the change in length of the lever.

It is particularly preferred that one uses a spacer between roller bearing and lever. This spacer can be adapted relatively accurately to the wavelength. By choosing a suitable Distanzstükkes can then cause the corresponding offset with sufficient accuracy.

Fig. 1

eine schematische Darstellung eines Kalanders,

Fig. 2

eine Möglichkeit zur Einstellung des Versatzes und

Fig. 3

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

The invention will be described below with reference to a preferred embodiment in conjunction with the drawings. Herein show:

Fig. 1

a schematic representation of a calender,

Fig. 2

a way to adjust the offset 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 formed as deflection rolls, and three center rolls 4 - 6, which together form a roll stack. The roll stack has a roll plane 7, in which the axes of all rolls 2-6 lie, when the rolls 2-6 are arranged exactly one above the other. In this roll level 7, for the purposes of the following description, also the press direction, i. the direction in which the rollers 2 - 6 are pressed against each other.

Further details of the calender are shown only schematically, such as a drive 8, or omitted entirely, such as means for heating individual rolls. However, the two end rollers 2, 3 and the middle roller 5 have an elastic coating 9, which is shown exaggeratedly thick.

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

In operation, it can now be observed that surface patterns form on the soft rolls which, when they have exceeded a critical size, are impressed into the paper web. In order to avoid such a barring pattern or to rebuild an existing barring pattern, it is now provided that the respective roller, in the present case the middle roller 5, is offset by a distance X from the roller plane 7. The procedure for this is to be explained in connection with FIG.

The roller 5 is mounted on a lever 14 which is suspended in a hinge, not shown, of the calender hinged. Here, the roller 5 has a bearing housing 15 which can be screwed to a bearing surface 16 of the lever 14 or fastened in some other way. In this case, the roller 5 is in the position shown in dashed lines. If it is now found that forms a barring pattern on the surface in this position, then the roller 5 is offset by the distance X by a spacer 17 is inserted between the bearing surface and the bearing housing 15. For this purpose, the bearing housing 15 is released from the bearing surface 16, the spacer 17 is inserted and the bearing housing 15 is again attached to the lever 14. The thickness of the spacer 17 is then the distance X. This distance X can be adjusted with a relatively high accuracy. The determination of this distance X will be explained with reference to FIG.

Shown in FIG. 3 are the roller 5, the roller 4 located above it, and the roller 6 located thereunder.

With excessively large amplitudes different reference ripples are shown, namely a ripple, in which seven waves run around the circumference of the roller 5, one with eight waves and one with nine waves. The numbers n = 7, 8, 9 were chosen for reasons of clarity. In the case of real rolls, correspondingly more waves will be set over the circumference of the roll, for example of the order of magnitude of 30 to 50. With so many waves running around the circumference of the roll 5, it can be assumed in the first approximation that with a small roll Offset movement of the roller 5 with respect to the roller plane 7, which is smaller than a wavelength, the curvature of the roller 5 is irrelevant.

When a barring pattern having a wavelength λ has occurred, the roller 5 is now displaced with respect to the roller plane 7, i. with respect to the nips 11, 12 so that the distance between the two nips 11, 12 on the one side increased by half a wavelength λ / 2 and on the other side by half this wavelength λ / 2 is reduced. For this purpose, only an offset X is required, which corresponds to X = λ / 4, because this results in the desired path length difference between the two nips 11, 12.

With a path length difference of λ / 2 arises at the points of the circumference of the roller 5, which have previously been heavily loaded and where accordingly troughs have formed, no load. This burden arises rather on the wave crests, where previously the corresponding burden was missing. The burdens resulting from the oscillatory movements of the three rollers 4, 5, 6 relative to each other. It can be achieved by a path length difference of λ / 2 so that an already trained barring pattern again imprinted and disappears over time. Although one risks that another barring pattern is formed whose wavelength is close to the wavelength of the original barring pattern. Thus, if the original barring pattern had a wavelength U / n, where U is the circumference of the roller 5, then the new barring pattern may have a wavelength of U / (n ± 1). But until such a new barring pattern is so pronounced that it bothers, some time passes.

If the barring pattern is not yet so pronounced that it bothers, or if one uses a replacement roll that is identical to the original roll on which the barring pattern has been shown, then one can also choose a roll offset X, which is exactly half the size, namely X = λ / 8. In this case, the path length difference is changed by the amount λ / 4. In the case of this path length difference, the previously generated barring pattern will not increase further, or no new barring pattern with this wavelength will be formed.

Due to the static roller displacement, which causes a path extension between the two nips 11, 12 of a quarter wavelength λ / 4, it can be assumed that at the same excitation by the coupling with the two adjacent rollers, the interference separated imprint at half intensity, so that theoretically a doubling of the service life can be achieved.

In practice, it will usually not be possible to achieve that rolls are actually identical in all parameters, even if a replacement roll is identical in construction to the roll to be replaced. Nevertheless, it can be assumed that a prolonged service life of the replacement roller can be achieved by a static roller offset, which is based on the wavelength of the barring pattern on the surface of the roller to be replaced, if, for example, the replacement roller is shifted by one-eighth wavelength λ /. 8 offset to the roller level 7, so that the distance between the two nips increased by λ / 4 on one side and reduced on the other side. Even if the replacement roller does not react with exactly the same response to the vibrations of the calender, one can expect an extension of the service life compared to a simple replacement without such a measure.

Claims (8)

1. Process for operating a calender having a roll stack which has two end rolls and a plurality of intermediate rolls between them, which bear on one another in a pressing direction, at least one roll being formed by a soft roll having a resilient surface, characterized in that, in the event of a barring pattern occurring on the circumference of the soft roll, a wavelength of the barring pattern is determined and a soft roll is offset transversely with respect to the pressing direction as a function of this wavelength.
2. Process according to Claim 1, characterized in that the roll is offset by an amount which is smaller than the wavelength.
3. Process according to Claim 1 or 2, characterized in that the roll on which the barring pattern occurs is offset.
4. Process according to one of Claims 1 to 3,
   characterized in that the roll is offset such that, on the surface of the roll, the effect is a path length difference of half a wavelength in a first case and of a quarter of a wavelength in a second case, in the first case a barring pattern having occurred on the surface of the roll and is to be eliminated and in the second case elimination not being required.
5. Process according to Claim 4, characterized in that the roll is offset by half the path length difference if the roll is an intermediate roll.
6. Process according to Claim 4 or 5, characterized in that a path length difference of a quarter of a wavelength is set when a roll is replaced by an identical replacement roll.
7. Process according to one of Claims 1 to 6,
   characterized in that the roll is offset by changing the length of a lever on which the roll is mounted.
8. Process according to Claim 7, characterized in that a spacer is inserted between roll bearing and lever.

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