WO2004005615A1

WO2004005615A1 - Method and apparatus for damping vibrations of a blade beam of a coating station

Info

Publication number: WO2004005615A1
Application number: PCT/FI2003/000542
Authority: WO
Inventors: Rami Vanninen; Jukka Koskinen
Original assignee: Metso Paper Oy
Current assignee: Valmet Technologies Oy
Priority date: 2002-07-04
Filing date: 2003-07-03
Publication date: 2004-01-15
Anticipated expiration: 2005-01-04
Also published as: AU2003238137A1; DE10392840B4; FI112878B; FI20021318A0; DE10392840T5

Abstract

The invention relates to a method and assembly for controlling the vibrations of a doctor blade beam. To the doctor blade beam (1) is adapted at least one dynamic damper assembly comprising at least a springed element (6) affixable to the doctor blade beam (1) and a mass supported to the springed element, whereby the natural frequency of the damper assembly is tunable to the natural frequency of the doctor blade beam.

Description

Method and apparatus for damping vibrations of a blade beam of a coating station

The invention relates to a method according to the preamble of claim 1 for controlling vibrations of doctor blade beams used in the coating of paper and paperboard webs. Such doctor blade beams support a doctor blade or rod that levels a coating mix applied to the surface of a moving web.

The invention also relates to an assembly suitable for implementing the method.

The printability qualities of a paper or paperboard web can be improved by different kinds of coats. Generally, the coating mix is applied to the web by a suitable application technique, whereupon the applied coating mix is leveled and doctored to the final coat weight by a doctor blade or rod. To a certain extent, doctoring may also be carried out, e.g., in paperboard manufacture using an air knife, while the limited doctoring effect of an air knife excludes its use in most cases at present running speeds.

Inasmuch as the coating layer is rather thin and on the other hand must be made very smooth to offer maximally good printability of a printing pattern on the surface of the paper web, the distance between the doctor blade and the moving web must be kept sufficiently accurately at a predetermined value. To this end, the position of the doctor blade and the quality of coating mix layer adhering to the web surface are monitored on-line by various kinds of measurement equipment. Based on these measurement results, the doctor blade position can be adjusted to obtain an optimal end result. Although the blade position is today easy to control by a variety of different techniques, continual increase in the web running speed and particularly in the machine width may cause problems in the management of doctor blade beam vibrations. If the doctor blade beam is allowed to vibrate especially at one of its eigen- frequencies, the coat will become uneven. Hence, there is a need for controlling the start of vibrations so as to prevent the doctor blade beam from entering vibrations of an increasing amplitude at its natural frequency. In principle, the natural frequency of the doctor blade beam could be controlled and attenuated by means of additional weights placed to the center of the beam. The weights must be very heavy, however, to attain effective attenuation. Yet this is not possible due to the limited space about the beam. Moreover, as a heavy weight placed in the middle of the beam increases the sag of the beam, it becomes necessary to make the beam sturdier or increase the deflection compensation of the doctor blade beam. These constraints complicate the dimensioning of the doctor blade beam and in a certain amount curtail the maximum possible cross-machine width of the doctor blade beam.

Doctor blade beams used today are supported at their ends by a pivotal joint to the coater station frame and at their middle point to the doctor blade beam cross tube of the coater station. This kind of construction has plural eigenfrequencies of which the one having the lowest natural frequency in the direction of the web travel is extremely harmful as to the result of the coating process because of its most drastic effect on the mutual position between the doctor blade and the web. The vibrations of the doctor blade beam are evoked by run-time cyclic excitations imposed on the beam during the coating process either via the foundation of the coater machine or generated by the rotational movement of the own rolls of the coater machine. The major source of excitation is formed by the rotating rolls when their speed of rotation coincides with the lowest natural frequency of the doctor blade beam in the direction of the web travel. At the coincidence of roll rotating speed with the natural frequency of the doctor blade beam, the beam begins to oscillate at an increasing amplitude eventually resulting in wide variations in the coat weight on the surface of the paper or paperboard web. With an increased width of the doctor blade beam, the natural frequency of the beam decreases and, in combination with the ongoing trend of increasing the machine speed, machines having a width of 8 to 10 m and running at speeds over 1200 m/min invokes operation in a range wherein the exciting frequencies and the natural frequency of the doctor blade beam may coincide. Moreover, the amplitude of natural frequency oscillations evoked in a wide beam may grow more drastic than those of a narrower beam excited by the same source. While the appearance of this kind of excitation could be prevented by way of changing the running speed of the papermaking machine, this is not generally desirable inasmuch as the goal is to run the machine at a maximum possible speed yielding the optimal or an at least acceptable quality of web coating. Hence, it is desirable to attenuate the vibrations of the doctor blade beam in order to eliminate possible resonant vibrations of the doctor blade beam.

It is an object of the present invention to provide a method capable of attenuating oscillations at the natural frequency of a doctor blade beam.

The goal of the invention is attained by virtue of adapting on the doctor blade beam at least one dynamic damper assembly comprising at least one springed element and a mass suspended by the springed element. Furthermore, the damper assembly is advantageously placed at the middle of the doctor blade beam and the springed element is complemented with damper element suitable for limiting the amplitude of the movements of the springed element.

More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1.

Furthermore, the assembly according to the invention is characterized by what is stated in the characterizing part of claim 5.

The invention offers significant benefits.

By virtue of the dynamic damper assembly, the vibrations of the doctor blade beam can be substantially attenuated. The mass to be actuated is smaller than that needed if the mass is mounted directly to the doctor blade beam and yet the novel arrangement gives a higher attenuating effect. The motion of the damper assembly can be constrained with the help of a hydraulic, pneumatic or frictional damper element so as to prevent an uncontrolled increase of the damper movement amplitude. As the vibrations of the doctor blade beam are thus kept at a low level, the outcome of the coating application is improved and the quality of the thus manufactured product becomes more uniform. Owing to the present invention, vibrations do not prevent increase of web speed even on wider machines. This is an essential benefit inasmuch as increasing the width of machines from present standards is complicated and costly, while on the other hand higher web speeds offer a substantially easier way of achieving a greater production capacity.

In the following, the invention will be examined in greater detail by making reference to the appended drawings showing a diagrammatic view of an embodiment of the invention.

Referring to the diagram, the support to a doctor blade beam 1 implemented is in a conventional fashion by having the ends of the doctor blade beam 1 mounted in bearings that pivotally support the beam to the frame of the coater. The doctor blade is mounted between the pivotal support points. As the doctor blade beam 1 may deflect under loading and its own weight, the pivotal support points in the bearings are implemented using linkage joints such that the support point as well as the end of beam 1 can rotate transversely in regard to the longitudinal axis of the beam. Accordingly, using the terms of static constructions, beam 1 is equipped with pivotal support joints at its both ends. To the middle point of beam 1 is adapted a screw jack serving to support the beam centrally and making it possible to rotate the beam according to the desired blade angle and changes in blade loading.

In the diagram is shown a portion of the doctor blade beam at the middle of the beam. The doctor blade beam is denoted by reference numeral 1. Exactly at the middle of beam 1 in the cross-machine direction thereof is adapted a mounting bracket 3 for an angle adjustment j ack 2 of doctor blade beam 1. The rod of the angle adjustment jack 2 is connected to the mounting bracket by a pivotal joint 4. Mounting arms 5 of the dynamic damper assembly elements extend downwardly from the mounting bracket 3. The dynamic damper assembly comprises two damper elements placed symmetrically about the mounting bracket 3. Either one of the damper ele- ments includes a bar 6 acting as a leaf spring element connected by its first end to the damper element mounting arm 5, a movable mass 7 and a damper member adapted between the second end of leaf spring bar 6 and doctor blade beam 1. The damper element is connected to doctor blade beam 1 by a mounting fixture plate 9.

Inasmuch as the greatest amplitude of vibrations in a doctor blade beam supported at its ends occurs at the middle of the beam, obviously also the optimal location of a dynamic damper assembly is at the middle of the beam. However, division of the damper assembly into two parts reduces the space required by the damper means and makes it easier to implement. Naturally, the damper assembly of the kind discussed herein is most advantageously made symmetrical. Obviously, the natural frequency of the damper assembly must be tuned to the natural frequency of the doctor blade beam. Variables in the computation of the damper parameters are the magnitude of the vibrating mass and the distance thereof from the support point. If the mass is made large, the natural frequency peaks of the damper are separated wide apart from each other, whereby the damper tuning by placing the mass at a correct distance from the support point is easier. On the other hand, a large mass added to the own mass of the beam increases the loading of the doctor blade beam and increases the space requirement of the damper. However, use of a small vibrating mass brings the natural frequency peaks closer to each other thus making the tuning of the damper on the natural frequency of the doctor blade beam more difficult. Due to these reasons, the magnitude of the mass has an optimum at about 20 % of the mass of the doctor blade beam. Then, the mass of either damper element is selected to be about 10 % of the doctor blade beam mass.

If the exciting force stays at a constant frequency coinciding with the natural frequency of the doctor blade beam and the damper, the amplitude of the damper movement grows without limitation. To keep the amplitude of vibrations within reasonable limits, an a kinetic-energy-absorbing damper device 8 is mounted to the end of the leaf spring bar 6 that acts as the springed element for the mass 6. The function of this absorbing damper device is to convert the kinetic energy of the dynamic damper assembly into heat by way of damping the movement of the end of leaf spring bar 6. The absorbing damper device may be any conventional device such as a hydraulic, frictional or gas-filled damper. A variety of suitable absorbing damper devices are available commercially, and also the design of such a device for the purpose is well known to a person versed in the art.

While the above-described dynamic damper assembly may be dimensioned relatively accurately for the correct natural frequency already during the design stage of the doctor blade beam, the best result will be attained by tuning the damper to the frequency of maximal attenuation during actual operation of the doctor blade beam. Then, the damper natural frequency coincides maximally accurately with the doctor blade beam natural frequency, whereby a damper design causing the damper to oscillate at an inverted phase relative to the doctor blade beam vibrations leads to efficient damping. Tuning can be performed by changing the distance of mass 7 from the support point, that is, the damper element mounting arm 5. In the exemplary embodiment shown in the diagram, mass 7 is divided in two parts that are adapted about leaf spring bar 6. The mass can be moved by undoing screws tightened in fixing holes 10, whereby the adjustment of the springedly responsive length of the springed element is possible. In this kind of damper, the final tuning of the damper may be carried out by moving the mass in a running coater and, if necessary, the tuning operation may be repeated if changes are made in the coater or doctor blade beam construction due to servicing or revamping. Furthermore, it is possible to adapt the mass movable by a threaded screw, for instance, whereby the tuning can be per- formed automatically based on, e.g., measurement of doctor blade beam vibrations.

In addition to those described above, the present invention may have alternative embodiments.

Advantageously, the springed element of the damper assembly is a leaf spring implemented as a rod-like element that is cost-efficient to fabricate and easy to dimension. Nevertheless, as the scope and spirit of the invention is not limited as to the construction of the spring, also a coiled spring for instance supported to the doctor blade beam may be used as the springed element. This kind of construction, however, generally requires more space than the simple damper used in the exemplary embodiment. While the damper could be placed in lieu of the middle of the doctor blade beam also at another point of beam, its damping efficiency may become weaker or require the use of larger masses since the amplitudes of vibrational movements at other points of the beam are smaller thus offering a minor chance of damping by vibrations of inverted phase.

The natural frequency of the damper must only be tuned to within the resonant frequency band of the doctor blade beam natural frequency, not necessarily exactly at the natural frequency of the doctor blade beam. Herein, this situation must be understood to cover the resonant frequency band within which the damper natural frequency is so close to the doctor blade beam natural frequency that the efficiency of damping is 50 %. If the damper natural frequency can be tuned very close to the doctor blade beam natural frequency, the amplitude of vibrations can be attenuated as small as one third of the undamped amplitude of vibrations.

Claims

What is claimed is:

1. A method for controlling the vibrations of a doctor blade beam of a coater station in a support assembly, the doctor blade beam (1) having a given natural frequency, characterized in that to the doctor blade beam (1) is connected at least one dynamic damper assembly comprising at least a springed element (6) affixable to the doctor blade beam (1) and a mass supported to the springed element, whereby the natural frequency of the damper assembly is tuned on the natural frequency region of the doctor blade beam.

2. The method of claim 1, characterized in that the dynamic damper assembly (6, 7, 8) is adapted to the middle of the doctor blade beam (1).

3. The method of claim 1 or 2, characterized in that the dynamic damper assembly (6, 7, 9) is divided into two damper elements adapted symmetrically about the middle point of the doctor blade beam (1).

4. The method of any one of foregoing claims, characterized in that the movement of the dynamic damper assembly is attenuated by means of a kinetic-energy- absorbing damper device.

5. An assembly for controlling the vibrations of a doctor blade beam of a coater station, the assembly comprising a doctor blade beam (1) having a given natural frequency, characterized by at least one dynamic damper assembly adapted to the doctor blade beam (1), the dynamic damper assembly comprising at least a springed element (6) affixable to the doctor blade beam (1) and a mass supported to the springed element, whereby the natural frequency of the damper assembly is on the region of the natural frequency of the doctor blade beam.

6. The assembly of claim 5, characterized in that the dynamic damper assembly (6, 7, 8) is adapted to the middle of the doctor blade beam (1).

7. The assembly of claim 5 or 6, characterized in that the dynamic damper assembly (6, 7, 9) is divided into two damper elements adapted symmetrically about the middle point of the doctor blade beam (1).

8. The assembly of any one of claims 5 - 7, characterized by a kinetic-energy- absorbing damper device such as a frictional, hydraulic or gas-filled shock absorber connected to the dynamic damper assembly.

9. The assembly of any one of claims 5 - 8, characterized in that at least one of the masses of the dynamic damper assembly (6, 7, 9) is adapted movable supported to the springed element (6) so as to implement an adjustable springedly responsive length of the springed element.