Method and arrangement for preventing oscillation in the roll group of a paper/board machine

The present invention relates to a method for preventing vibrations in the roll group of a paper/board machine comprising at least one deflection- compensated roll and a backup roll forming a nip with it, the deflection- compensated roll including a stationary central shaft and a roll shell arranged to rotate around it, the said roll shell being supported on the shaft by means of loading elements loadable with a pressure medium and by means of backup-zone elements loadable with a pressure medium and forming at least one backup zone, by means of which loading elements and backup-zone elements the magnitude of the line load acting on the nip plane is controlled.

In roll groups, such as multi-roll calenders, vibrations may be generated that affect the quality of the fibrous web being processed, especially when the running conditions (incl. nip pressure, humidity of the fibrous web, speed of the fibrous web) have been constant over a longer period.

An earlier application EP1704280 by the applicant of the present application discloses a method and arrangement for preventing vibrations in a multi-nip calender by continuously or intermittently changing the running parameters acting on the calendering impulse of the roll nips so that the overall calendering impulse of the calender or calender stack remains substantially constant or within predetermined limits. In an earlier application WO00/09805 by the applicant of the present application is disclosed a solution based on the use of a pressure accumulator for changing the specific frequency of the roll structure by varying the elasticity of the hydrostatic sliding bearing.

The aim of the present invention is to provide a solution by means of which the properties of a single deflection-compensated roll belonging to a group of rolls consisting of two or more rolls can be changed by internal means of the

roll to prevent vibrations in the roll group. To achieve this aim, the method according to the invention is characterised in that the pressure levels of the loading elements and backup-zone elements of a deflection-compensated roll are varied at desired moments, so that the rigidity of the roll shell will change while the line load acting on the nip plane remains essentially as desired. The arrangement according to the invention, on the other hand, is characterised by what is disclosed in the characterising part of independent claim 9.

The inventors of the present invention unexpectedly found that, contrary to general theories on the rigidity of the hydraulic system, according to which the rigidity will not change even if the pressure changes, due to the constant compressibility of oil, the rigidity - and thus the specific frequency - of a back-up zone deflection-compensated roll will change when the pressure level is changed. This change in specific frequency following a change in the pressure level was found in experimental measurements. Without being bound by theory, it is assumed that the change in specific frequency is due to the fact that when the pressure is increased, the sealing friction apparently increases, the O-rings etc. are compressed more tightly, whereupon the rigidity of the roll increases and the specific frequency changes. However, the paper manufacturing process and the quality of paper require that when the rigidity of the roll is changed, the line load in the roll nip in the transverse direction of the paper web, that is, the line load profile, must not change. The line load is formed as the sum force of the loading elements acting in the direction of the nip and of the loading elements of the backup zone. The line load profile must be maintained unchanged when the pressure level of the row of backup-zone elements and of the row of elements acting in the direction of the nip are raised simultaneously in essentially the same proportion, that is, taking into account the different number of loading elements and the different surface area in the backup zone and nip element rows. When it is desirable to increase the rigidity of

the roll, that is, to increase the specific frequency, the pressure level is increased in the backup zone and nip element rows, and correspondingly, when it is desirable to lower the specific frequency, the pressure level in both or all element rows is decreased.

By means of the solution according to the invention, the rigidity - and thus the oscillating properties - of the roll shell of a deflection-compensated roll is relatively simple to adjust without affecting the value of the line load exerted on the fibrous web. The line load is set by utilising the same loading means and backup-zone means, but in changing the pressure levels of the loading means and the backup-zone means in essentially the same proportion, the value of the line load remains essentially at its set value while the bearing rigidity of the roll shell changes in the desired manner.

The invention is described in greater detail in the following, with reference to the accompanying drawings, in which:

Figure 1 shows a diagrammatic cross-sectional view of a prior art deflection-compensated roll with one backup-zone row, and

Figure 2 shows a diagrammatic cross-sectional view of another prior art deflection-compensated roll with two backup-zone rows,

Figure 3 shows a deflection-compensated roll provided with two backup- zone rows as a diagrammatic vertical view in longitudinal section, showing a hydraulic medium feeding arrangement for the loading elements, and

Figure 4 shows a roll corresponding to Figure 3 as a diagrammatic, horizontal, longitudinal section.

The prior art deflection-compensated roll 1 shown in Figure 1 comprises a stationary central shaft 7 and on it a roll shell 2 arranged to rotate around it. Between the central shaft 7 and the roll shell 2 loading elements 3 are fitted acting on the inner surface of the shell 2 and on the nip plane 6 backup-zone elements 4 acting in the opposite direction. The end bearings of the roll and other accessories, as well as more detailed structural details of the roll, have been omitted from the figures for the sake of clarity. In the longitudinal direction, the roll 1 comprises one row of spaced loading elements 3 and one row of spaced backup shoes 4.

Another prior art deflection-compensated roll 1 shown in Figure 2 also comprises loading elements 3 as well as backup-zone elements 4A, 4B fitted between a rotating shell 2 and a stationary central shaft 7. There are backup-zone elements 4A, 4B, or backup shoes, in two rows along the longitudinal direction of the roll 1, arranged in each row successively at a distance from one another. The backup-zone element 4A, 4B may also be integrated. The backup zone may also be comprised of a backup chamber or of a combination of a backup chamber and backup shoes. The angle α between the rows of backup shoes or integrated backup shoes 20, 21 is 0 < a < 180° and the angle α selected so that the deformations/ stresses can be optimised. This type of solution is disclosed, for example, in the publication EP 0764790 Bl.

Figure 3 shows a deflection-compensated roll 1 and its backup roll 100. The backup roll shell 102 may be of metal or of a composite material, or it may be an elastic belt (a so-called long-nip press roll). The roll 1 is fitted with roller or slide bearings 50. The loading elements of the roll 1 are marked with reference numeral 40. The figure shows that each loading element 40 provides an independent zone Zi; Z2 ... Z _n , but each zone could equally well consist of several elements. The hydraulic medium supplied to all zones is delivered along a single pressure line 15 either directly to the roll shaft 7 or

to a valve block 20 connected to the shaft. The hydraulic medium is guided through the valves 16 to each zone Zi, Z ₂ ... of roll 1 along corresponding pressure lines zi, z ₂ ... The pressure of the hydraulic medium in the feed line may be, for example, of the order of 120 bars.

In the roll 1 there is a backup zone C _z acting in the opposite direction of the nip, the said backup zone comprising backup-zone rows formed by loading elements 160a, 160b on opposite sides of the nip plane 6, cf. Figure 4. Loading elements 160a are connected to a joint pressure line 161 and loading elements 160b to a joint pressure line 162. The backup zone C ₂ generally has a single zone, which means that the same pressure is supplied via pressure lines 161, 162 to all its loading elements 160a, 160b, but it may be profiling in the nip direction, for example, a 3-zone one. Also in the case of a profiling backup zone, it is preferable that the same pressure prevails at each zone on opposite sides of the nip plane to prevent the risk of lateral forces bending the shaft. It is possible to integrate the valves of the backup zone C ₂ in the valve block 20.

The hydraulic medium required by other applications, such as bearings and gearing, may be supplied to the roll in the conventional manner via separate pipes or tubes. However, preferably all the hydraulic medium required for the roll is supplied through a single pressure line 15 to the block 20 connected to the roll shaft 2 and then guided, for example, through pressure reducing valves or other regulating/control valves to the different applications, such as loading elements, bearings, gearing and a jet tube. For example when the bearings 5 are slide bearings, the hydraulic medium required by them may be supplied either directly to the bearing block or shaft, or via the line 15 to the valve block 20, from where it is led to its target.

By changing the pressure levels of the loading elements 40 acting in the direction of the nip and of the backup-zone elements 160a, 160b in the same

proportion in accordance with the invention, the set line load will not change essentially, but the rigidity of the roll shell and at the same time its specific vibration frequency will change, whereby the falling of the roll vibrations into sync with the speed of rotation of the roll can be prevented, which will most probably take place when the running conditions remain essentially the same over long periods. During long runs with the same power settings, that is, with the same specific frequency of the roll, the coating of a deflection- compensated roll with a polymer coating is deformed into particular form dependent on the specific frequency (round pointed star with specific wave number), which acts as excitation to vibrations. When the specific frequency is changed, the deformation of the coating, that is, the wave number is mixed up and the vibration is not amplified.

A particular profile for the line load in CD-direction can be achieved with many different pressure level distributions. The line load is calculated preferably by using an optimisation programme, whereby the pressure levels can be changed relatively simply by changing the optimisation parameters used by the optimisation programme for calculating the line load.