The invention relates to the bearing assembly for a roll forming a nip, comprising a bearing housing that can be attached to the structures of a web treatment machine, a bearing enabling rotation of the roll, comprising a peripheral element in contact at least with the bearing housing, and an opening for the roll shaft, the roll apt to be disposed in the bearing assembly being adapted both for rotation and for movement in parallel with the nip load line and transversely to the nip load line. The invention also relates to a method for damping roll vibrations in the nip between compressed rolls in a stack of rolls of a web treatment machine, at least one of the rolls being deflected transversely from the nip load line of the roll stack.

Vibration phenomena occur in web treatment machines, such as calendering machines, the rolls pressed against each other in a set of rolls starting to vibrate and causing depressions in the soft roll coatings. Such depressions produce a corresponding whip-like pattern in the web to be treated. The paper industry journal Wochenblatt fur Papierfabrikation, Nr 4, 1976, Gϋnther Boos, "Die Ursache von Glattwerksmarkierung (Barring) im Papier und deren Behebung durch den Einsatz der Escher Wyss - Nipcowalze", pp. 133-139, contains an extensive article on vibration phenomena marking the web during calendering, i.e. barring, and the mechanisms of its action and occurrence. The discussion section of the article on page 139 says, among other things, that barring can be prevented by a lateral shift, i.e. transversely from the loading direction, of about 10-40 mm of one roll in a roll stack during operation.

In the context of this invention, nip load line or load line in general implies, with moderate precision, the direction corresponding to the line passing through the axes of rotation of at least two adjacent rolls in nip contact in a web treatment machine, or the direction defined by the line between the axes of rotation of the outermost rolls of rolls in mutual nip contact forming a roll stack, i.e. the upper and lower roll in a conventional supercalender. In the latter case, relating to a roll stack, the load lines defined by two adjacent rolls in the roll stack and the outermost rolls in the roll stack are not necessarily aligned.

WO 99/25921 is a prior art reference disclosing a method for detecting contamination or damage of a roll surface passing through a nip. In accordance with the method, vibrations occurring in the structure are detected and analysed, and then the necessary measures can be taken in order to eliminate vibrations.

EP 1015695B1 is also a prior art reference disclosing a method and a device in a roll assembly in a paper machine or a paper-finishing machine, especially for damping vibrations generated in a nip between a pair of rolls pressed against each other without changing the running speed of the machine. The arrangement is based on a dynamic vibration absorber, i.e. on adaptation of the oscillation frequency of the movable mass at the end of a bar to a value as close as possible to the specific frequency of the rotating roll, and then this movable mass oscillates at the same frequency, yet in the inverse phase, thus damping harmful vibrations proper. Since the oscillation frequencies result from very complex situations, such as e.g. a spring action generated by a depression caused in the roll coating by a seam or any web irregularity passing through the nip, the dynamic vibration absorber needs to have a frequency adaptable specifically to the oscillation frequency occurring each time.

US 2002/0024415A1 is also a prior art reference disclosing vibration dampening in a calendar roll stack as proposed by Boos, with one of the rolls in a stack of rolls pressed against each other deflected into a direction transverse to the compression direction. The reference mentions that preferably the vibration level of the central roll in a stack of rolls is monitored and analysed, and on the basis of this analysis, the wavelength of the occurring or generated vibration is determined relative to the roll circumference or the multiple of the circumference. By subsequent deflection of the roll transversely from the compression direction of the roll stack, the oscillation pattern thus produced is altered to such an extent that the periodical whip-like pattern generated in the soft roll coating is not allowed to increase detrimentally. According to the reference, the roll is preferably shifted laterally over a distance equalling one eighth - one quarter of the determined wavelength. The reference suggests that the lateral shift be performed by means of an eccentric, a linear guide, a folding lever mechanism or a spindle screw.

The invention has the aim of achieving a roll bearing assembly allowing the implementation of the teachings set forth by Gϋnther Boos in his article in 1976 and allowing easy, economical and simple reduction of the tendency of compressed rolls to start vibrating, since such vibrations may result in irregularities in the web passing through the nip between compressed rolls. The invention also has the purpose of notably simplifying the structures disclosed by US 2002/0024415A1 in the bearing assembly of a roll that is movable transversely to the compression direction of the calender. The invention has the additional purpose of providing an opportunity of consistently modifying the calendar geometry, allowing prevention of vibration phenomena, which often start appearing gradually.

The invention also has the objective of acting on the bearing spring constants, which are also mentioned in the Boos article, and which contribute to the vibration characteristics of the web treatment machine as a whole.

The bearing assembly of the invention is characterised by the fact that at least one space has been provided between the bearing housing and the peripheral element, the space being located at a distance from the load line and capable of receiving a supply of fluid pressure hydraulic medium, which serves to deflect the roll transversely from the nip load line by shifting the peripheral element relative to the bearing housing.

The method of the invention, in turn, is characterised by the fact that, in the roll bearing assembly, at least one space located between the bearing housing and the peripheral element at a distance from the load line is supplied with a fluid pressure medium, by means of which the roll is deflected transversely from the nip load line by shifting the peripheral element relative to the bearing housing.

This proceeding achieves a simple and reliable bearing assembly and vibration damping method, which comprises a minimum of components and enables the geometry of the web treatment machine to be modified during operation so as to prevent any detrimental vibration phenomena and to stop any vibration at its initial stage.

According to a preferred aspect of the invention, the bearing assembly comprises means for determining the location of the peripheral element relative to the bearing housing. Consequently, the current state of the structure will be known at each moment, ascertaining consistently that the shift has been identical at both ends of the roll, i.e. that the roll remains straight and in the desired position relative to the other rolls in contact with this particular roll.

According to a second preferred aspect of the invention, the peripheral element is the outer periphery of the actual bearing or a stiffening element connected to this outer periphery.

In conventional bearing assemblies, the bearing housing supports the bearing proper, so that all the forces exerted on the structure are transmitted from the rotating roll as desired to the support base of the bearing housing, i.e. to loading levers or the frame. In some cases, e.g. when the web treatment machine is a calender, the bearings may be subjected to very strong forces. The bearing proper does not necessarily have a design such that resists such a force when, as it were, compressed between two planar surfaces and being allowed to deform freely from a circular shape to an oval shape. This means that lateral deformation needs to be prevented, and it can be appropriately carried out by means of a separate stiffening element, which prevents undesirable deformation of the outer periphery, i.e. the rolling path of the bearing proper or the bearing element of a slide bearing. This is accomplished by giving the stiffening element a design such that allows transmission of the nip loading force from the bearing housing to the bearing, with the bearing deformation generated by the loading force remaining on a level lower than the one accepted for deformation of the bearing to be utilized.

The invention is explained below with reference to the accompanying drawings, in which:

Figure 1 is a side view of the general arrangement of a bearing structure, i.e. viewed transversely to the web movement direction, Figure 2a illustrates an optional path of the roll rotation axis, Figure 2b illustrates a second optional path of the roll rotation axis, Figure 2c illustrates a third optional path of the roll rotation axis.

Figure 1 shows the bearing assembly 2 of a web treatment machine, such as a calender, with the parts essential for comprehension of the invention. The bearing assembly 2 has the main task of allowing rotation of the roll 3 and of transmitting bearing forces necessary for the web treatment between the roll 3 and the frame of the web treatment machine. Owing to the invention, the bearing assembly receives an additional function: the bearing assembly allows active vibration damping to be implemented whenever necessary. The figure illustrates an oval opening in the bearing housing 4, the opening forming spaces 7 on the sides of the peripheral element 6 allowing transverse movement of the peripheral element 6 relative to the bearing housing 4. The space 7 is placed at a distance from the load line L. Figure 1 shows the roll 3, or the roll shaft 31 in the practice, located midway on its transverse path in accordance with the invention. The axis of rotation A of the roll 3 is preferably conducted to either end Cp1 , Cp2 of its path C for the structure behaviour to remain as stable as possible.

In figure 1 , the peripheral element 6, being e.g. the outer periphery 61 of the bearing or a stiffening ring 62, is illustrated as a ring with regular shape, yet in the practice, it may have a different design. Advantageous features for the function of the structure comprise adequate stiffness in order to transmit the necessary forces together with mobility relative to the bearing housing. Mobility may involve e.g. translation on a suitable surface or rotation about a conveniently selected point, or a combination of these. Thus, for instance, when a circular shape is selected for the peripheral element, mobility is conveniently provided by rolling the peripheral element 6 along the opening in the bearing housing 4. The use of a pressure medium, such as hydraulic oil or the like, naturally requires sealing to prevent the pressure medium from reaching locations where it would be harmful or from leaking out of the bearing assembly. The space 7 or spaces 7 are obvious objects of sealing when the roll is in position Cp1 or Cp2. Commonly used solutions are suitable as seals.

According to a preferred aspect of the invention, the bearing assembly comprises means 11 for determining the position of the peripheral element relative to the bearing housing. These means 11 may be e.g. optical, mechanical, inductive or other electric sensors, or similar instruments, allowing reliable determination of the current state and position of the

structure at each moment. The invention has the objective of controlling the construction, with the shift at both ends of the roll having been or being made identically, i.e. with the roll remaining straight and in the desired position relative to the other rolls in contact with this particular roll.

The roll is preferably movable with a movement frequency that can be selected by the operator. In one embodiment, the roll can be oscillated by means of the pressure medium transversely to the nip load line. The length of the movement, i.e. the distance between position Cp1 and position Cp2, depends on how the space 7 has been arranged. In one conceivable embodiment, the space 7 has been provided so as to allow the roll to be deflected from the nip load line by at least 10 mm, preferably 40 mm, or even more if necessary.

Since the mobility described above has an impact on the flexibility of the construction, which, in turn, influences the vibration behaviour of the structure as a whole, this flexibility should advantageously be controllable in terms of the construction. One means of control comprises locking means in the bearing assembly for locking the peripheral element in an immobile position relative to the bearing housing. This provides clearly better stiffening of the construction than merely using a pressure medium for locking the peripheral element 6 relative to the bearing housing 4.

In the vibration damping method, it is advantageous to monitor the position of the peripheral element relative to the bearing housing and to shift it whenever necessary by at least 10 mm, preferably 40 mm, transversely to the load line. To prevent the generation of roll vibration, the operating conditions are monitored and analysed, and a decision about modifying the geometry of the web treatment machine is taken on the basis of the analysis, the position of the roll relative to the load line being shifted consistently at selected intervals or less frequently. The operating conditions can be supervised e.g. by means of appropriately disposed acceleration sensors, optic sensors monitoring the surface of the web or the roll, or by any other suitable means. The data thus obtained are processed and analysed with a control system or a system connected to this. Most advantageously the system identifies a starting vibration as early as possible, so that its prevention will be as easy as possible. The decision about modifying the geometry of the web treatment machine is taken by

means of a program in the control system or at the operator's choice. As a whole, it is advantageous that the roll deflection could be switched off and the movement of the peripheral element relative to the bearing housing could be prevented by means of a mechanical coupling. This can be carried out under running conditions such as a given running speed, nip load, web material characteristics and composition etc. that have proved less apt to generate detrimental vibrations.

Figures 2a, 2b and 2c illustrate various options for disposing the path C of the rotation axis of the roll 3 relative to the nip load line. In figure 2a, the end point Cp2 of the path C is disposed on the load line L. The second end point

Cp1 of the path C is disposed at a suitable distance from the load line C. In figure 2b, the path C intersects the load line L and the end points Cp1 and

Cp2 are located on different sides of the load line L. In figure 2c, the path C does not intersect the load line L, both the points Cp1 and Cp2 being located on the same side of the load line. This arrangement yields the benefit of a very stable construction, yet it has the drawback of the roll shift having a smaller action on vibration, i.e. its technical effect is lower than in the arrangements shown in figures 2a and 2b. As shown in figures 2a-c, the path C may be straight or curved.

In one embodiment of the invention, the pressure medium comprises a magnetorheological (MR) fluid. Such fluids are characterised by the fact that their properties, such as viscosity, can be influenced by means of a magnetic field (M). This, again, is based on the fact that the fluids contain particles that are concatenated under the effect of the M field. This means that rheological properties can be influenced by other means than temperature variation alone. The viscosity variation may be quite significant, up to one decade when MR fluids are used. The embodiment of the invention can thus utilise the adaptive rheological properties of MR fluids, a change of the properties of the MR fluid by means of the magnetic field allowing action on the transverse movement of the roll 3 relative to the load line L. This rheological property is very useful e.g. in a situation where the roll is oscillated by means of a hydraulic medium transversely to the nip load line during web treatment. The MR fluid thus achieves a transfer movement that can be controlled in many ways, allowing far more versatile actions on the parameters of the oscillatory movement, such as speed and damping, compared to the use of a conventional hydraulic medium. As examples of

this, we cite rapid roll shift or particularly slow roll shift during web treatment. The use of MR fluids naturally requires the bearing assembly to comprise appropriate means for generating and controlling the magnetic field acting on the MR fluid.

Reference numerals in the figures:

1 web treatment machine, e.g. a calender

2 bearing assembly 3 roll

31 roll shaft

4 bearing housing

5 bearing

6 peripheral element 61 outer periphery of the bearing 62 stiffening element

7 space

L load line

A rotation axis of the roll 3 C path of the rotation axis of the roll

Cp1 position point 1 of the rotation axis of the roll Cp2 position point 2 of the rotation axis of the roll 11 means for determining the position