In paper production, rotating rolls are used between which the paper web is fed.

Roll vibration may cause problems, especially when increasingly higher rotation speeds are aimed at. Such problems arise particularly in nip rolls used for instance for wet pressing, coating, calendering and rolling of the paper web. Vibration shortens the service time of the rolls, increases power consumption, causes noise and impairs the paper quality.

The following elements may be in mutual contact in a nip: a metal roll/a polymer roll, a metal roll/a paper roll, a metal roll/a metal roll, a polymer roll/a polymer roll, a metal roll/a belt, etc. Calender rolls may be coated with various relatively soft coatings, such as for instance plastics, which may be damaged by vibration even over a very short period of time. In this conjunction, the coating is especially at risk of being corrugated in the peripheral direction of the roll. Any points of roughness are naturally transferred to the paper. The coating may also be detached from the roll. Especially in the case of calender rolls, vibration as such may also result in irregular paper quality. To eliminate vibration problems, the requirements on high- precision roll manufacture and especially roll balancing are continuously increasing.

Mechanical vibration dampers acting on the support points of the roll axis and placed for instance in bearing housings have been used to dampen vibration.

Hydraulic vibration dampers have also been suggested for use inside the roll mantle.

One such installation is disclosed in US specification 5,487,715.

Description of the invention The method of claim 1 has now been invented for damping the vibration of a roll used in paper production. Some embodiments of the invention are set forth in the other claims.

In accordance with the invention, damping is exerted by means of damping means externally on the roll mantle. In this way, the damping can be directed to the desired location, especially to the point where the vibration has the largest amplitude.

Normally the vibration amplitude is largest at the centre of the roll.

The damping means is preferably such that allows control of the damping. A plurality of damping means may also be provided.

The damping means may be in contact with the roll surface or detached from this.

In active damping, a vibration absorbing force is exerted externally on the roll e. g. with the aid of a magnetic field or a fluid jet. In this case, especially variation of the vibration damping force exerted on the roll allows control of the damping.

In passive damping, the vibration energy of the roll is eliminated by means of a damping means, e. g. a damping means pressed against the roll. In this case, the damping force depends on the relative kinetics of the damping means and the point of contact. The damping means may be supported on its own support frame so that a dampening effect is provided in the support arrangement. The damping effect can be achieved for instance by Coulomb or viscotic friction. This allows control of the vibration absorption of the roll by changing the damping of the support arrangement. The damping effect can also be provided in a means lying against the roll itself, this means being made for instance of a suitable elastic material such as sponge. The vibration energy of the roll is converted into heat in the damping means. The damping means can be pressed against the roll with a radial bar or a lever.

The damping action can be exerted only on a portion of the roll mantle, especially on its centre. Then the damping means may be such that is movable in the axial direction of the roll. The damping means may also be such that is movable in the peripheral direction of the roll. The damping means can also be programmed to move automatically under a specific program.

In a system comprising two rolls rotating in mutual contact, vibration damping is preferably exerted parallel to the plane passing through the axes of the rolls.

The roll is connected with a device for measuring vibration. The damping is controlled on the basis of the measurement results. When a movable damping means is used, the location of the damping means can be changed. The measurement results also allow control of the damping characteristics. This control

can be automated. The measurement can be based for instance on ultrasonic, laser or microwave technology. The measurement allows the vibration amplitude, frequency or phase angle to be monitored as a function of time.

A mechanical damping means pressed against the roll may act simultaneously as a means for cleaning the roll. It may comprise for instance a sponge moistened with a cleaning liquid. A fluid jet such as an air jet may also act as a damping means or a cleaning means.

When a magnetic cleaning means is used, the roll surface does not have to be touched. It may operate also through the paper web.

Vibration damping can be used especially in cases where rotating rolls are in mutual contact, as for instance in wet pressing, coating, calendering and rolling. The vibration frequencies in a calender, for instance, may typically be in the range 100... 1,000 Hz. In a coating nip, for instance, the frequencies may be even lower, as low as approx. 50 Hz.

Some embodiments of the invention are explained in detail below. The accompanying drawing pertains to the disclosure. The figure is a schematic front view of a calender comprising the vibration damping of the invention.

The calender shown in the figure comprises a lower roll 1 and an upper roll 2 mounted on bearings in a frame. Above the upper roll, a transverse beam 3 parallel to the roll axis is provided, with its ends fastened to its own support frame. A slide 4 movable in the axial direction of the rolls is connected to the beam in the plane passing through the axes of the rolls. The slide comprises an arm oriented radially towards the roll. The arm comprises a damping means 6 at its lower end.

The arm 5 is adjustable so as to allow the distance from the damping means 6 to the roll 2 to be varied.

The damping means 6 may be active, such as an electric magnet exposing the roll to a magnetic field.

The damping means 6 may also be passive, such as a mechanical press bearing against the roll. In that case, the support arrangement of the damping means comprises effective damping, which converts the roll vibration to heat. To achieve this, the arm 5 may be for instance a hydraulically operated shock absorber.

On the other side of the beam 3, a measuring device 7 has been disposed, which also moves in the transverse direction, and by means of which the roll vibration, for instance its amplitude, is measured. The measuring device is connected over a control system 8 to the control apparatus of the slide 4 and the arm 5. This allows the location and the damping force of the damping means to be controlled according to the signal emitted by the measuring device.