WO2004005614A1

WO2004005614A1 - Method and apparatus for controlling vibrations of a blade beam of a coating station

Info

Publication number: WO2004005614A1
Application number: PCT/FI2003/000541
Authority: WO
Inventors: Rami Vanninen; Petri Parni
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: DE10392825T5; FI20021319A0; DE10392825B4; AU2003242801A1; FI112879B

Abstract

The present publication discloses a method and assembly for controlling the vibrations of a doctor blade beam (1) of a coater station in a support assembly, wherein the doctor blade beam (1) is supported at its both ends by a pivotal joint (2) and further by at least one actuator means (3) for adjusting the position of the doctor blade beam (1). The doctor blade beam (1) is supported with the help of at least one member (4, 5) of controllable stiffness such that changing the stiffness of the support assembly controls the natural frequency of the doctor blade beam (1).

Description

Method and apparatus for controlling 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. 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. 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 control 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 readily shifting the natural frequency of the doctor blade beam aside from the exciting frequency. The goal of the invention is attained by virtue of changing the support of the doctor blade beam to include at least one member of controllable stiffness.

To this end, the doctor blade beam can be equipped with, e.g., at least one support suitable for stiffening the doctor blade beam on some other structure. Advantageously, the number of supports is two and they are adapted close to the ends of the doctor blade beam, in a close vicinity of the pivotal support points thereof.

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.

The method may be readily adapted to existing doctor blade beams by complementing the beam ends with lockable joints implemented as sliding joints, for instance. Generally, sufficient installation space can be found for such members in a close vicinity of the doctor blade beam. If the support members of controllable stiffness are adapted to the beam support assembly already at the design stage, a variety of different support arrangements can be contemplated. When the stiffness of the doctor blade beam support assembly is changed, simultaneously also changes the natural frequency of the beam thus offering the facility of shifting the natural frequency of the beam off from the frequency of the exciting cyclic force by virtue of increasing/decreasing the stiffness of the beam support system. In the simplest embodiment of the invention, the support assembly comprises a controllable joint that can be locked or released as required. As soon as the start of vibration of a growing amplitude is detected, the support or supports is/are locked, whereby the natural frequency of the doctor blade beam changes and the beam will not go into resonance at all or at least the resonance is substantially attenuated. Respectively, when the frequency of excitation reaches the natural frequency of the locked doctor blade beam, the locking can be released, whereby the natural frequency of the supported beam system is again shifted off from the impending resonance. It may further be contemplated to use a controllable locking force, whereby the natural frequency control range becomes wider. An adjustable support locking force may be implemented using, e.g., hydraulic or pneumatic means.

In the following, the invention will be examined in greater detail by making reference to the appended drawings in which

FIG. 1 shows a diagrammatic perspective view of an embodiment of the invention; and

FIG. 2 shows a support member suitable for implementing the invention.

Referring to FIG. 1 , the basic construction of a support assembly for a doctor blade beam 1 is shown therein diagrammatically. The ends of the doctor blade beam 1 rest on bearing blocks 2 via which the beam is pivotally supported to the coater machine frame. A doctor blade 18 is mounted between the pivotal support points. As the doctor blade beam 1 may deflect under both the loading imposed thereon and its own weight, the bearing blocks 2 are implemented as a pivotal joint so that the pivot point and, thus, also the end of beam 1 can rotate in a plane perpendicular 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 3 serving to support the beam centrally and making it possible to rotate the beam according the desired blade angle and changes in blade loading. The screw jack is mounted on a frame beam 17. As a result, the doctor blade beam 1 is pivotally supported at its ends, while the support provided at the middle point of the beam is capable of taking only vertical loads in a plane substantially parallel to the longitudinal axis of the beam.

In the exemplary embodiment discussed herein, the doctor blade beam 1 is supported at its both ends by lockable support members 4, 5. The natural frequency of the doctor blade beam can be controlled with the help of these support members. Either of the support members comprises a slide rod 19, a pivotal joint 15 connecting one end of the slide rod 19 to the doctor blade beam 1 and a lockable slide bushing 16 pivotally connected to the frame beam 17. When the lockable support member such as the sliding joint 4, 6 is controlled into its unlocked state, the movement of the doctor blade beam is unhindered, whereby sliding rod 19 glides freely in bushing 16 inasmuch as the pivotal joint 15 of the rod end and the joint of the slide bushing are freely rotatable. Sliding rod 19 can be locked into bushing 16 in plural ways using, e.g., a tapering joint, a wedged key or a compressive/eccentric clamp. The move- ments of such locking means may be implemented using conventional actuators based on, e.g., hydraulics, pneumatics or electrical devices. When the lockable support member is clamped immobile, also the play at its pivotal joints 15, 16 must be eliminated. This can be accomplished in the fashion described below.

Under a normal operating situation involving no risk of resonance, the locking of the support members is kept in their released state, whereby they provide no support to the doctor blade beam. As soon as the resonant frequency is approached, support members 4, 5 are locked thus giving doctor blade beam 1 a stiffer support and increasing its natural frequency. Next, when the exciting frequency approaches the new natural frequency value of the stiffened doctor blade beam, the locking of support members 4, 5 is released, whereby the natural frequency of the doctor blade beam shifts to a lower frequency. In this fashion the natural frequency of the beam can be shifted off from the exciting frequency under all conditions so as to avoid a resonant state of the doctor blade beam 1.

The support members must be constructed such that they can be controlled unlocked during the rotation of the doctor blade beam 1 and, moreover, causing not obstruction to the movement of the doctor blade beam 1. One embodiment of such a support member is illustrated in FIG. 2. The support member shown in the diagram is a lock- able pivotal sliding joint 6 comprising an outer tube 7 and a rod 8 adapted to slide therein. Normally, the rod 8 is free to move in the outer tube 7, but is locked immobile when the sliding joint 6 is locked. The above arrangement provides stiff support at the ends of doctor blade beam 1. The ends of outer tube 7 and sliding rod 8 are provided with mounting elements 9, 10 fixed on bushings 11, 12 mounted on doctor blade beam 1 and at a suitable fixed support point such as the coater station frame, for instance. Mounting elements 9, 10 and bushings 11, 12 have concentric bores wherein tapering pins 13, 14 are inserted. The tapering pins 13, 14 serve to eliminate the play of the sliding joint 6 at its fixing points and to provide stiff support to the beam. The control of the movement of the tapering pins is synchronized to the locking control of the sliding joint 6 so that in order to change the angle of the doctor blade beam, the lockable sliding joint is released with simultaneous slight withdrawal of the tapering locking pins, whereby a play at the fixing points is provided allowing the fixing elements of the sliding joint to rotate with the rotation of the doctor blade beam. The functions of the sliding joint and the tapering pins can be implemented hydraulically inasmuch as a coater station generally has ample hydraulic power available. Naturally, the functions may also be realized using electrical or pneumatic actuators.

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

In lieu of the above-described mechanical sliding joint, a double-acting hydraulic cylinder could be employed, whereby its locking takes place through closing the hydraulic fluid channels of the cylinder chambers. While also the use of pneumatic actuators could be contemplated, their compressibility may cause problems. Yet, also this kind of actuator may be equipped with mechanical locking if so desired. It is further feasible to make the stiffness of the support force controllable if a large number of vibration-exciting sources or a wide exciting frequency range thereof must be managed. However, excitation of vibrations normally appears on certain fixed frequencies only, whereby a simple support assembly with a lockable/releasable stiffness is a sufficient solution and cost-efficient to implement. Frictional, hydraulic or pneumatic techniques can be employed to realize a controllable support member. Instead of adapting the support member of controllable stiffness to the beam ends, also other locations along the beam may be contemplated. However, the support assembly according to the above-described exemplifying embodiment is easy to adapt in place and, moreover, the support system becomes symmetrical. Alternatively, a support system could be used aligned in the direction of the center axis of the beam and acting on the beam ends so as to constrain the rotation of the beam end. Respectively, the rotation of the pivotal joint of the beam end can be constrained by a support assembly with a controllable frictional force or lockable rotation. While obviously plural support members could be contemplated, the benefit of having two support members placed symmetrically about the middle point of the doctor blade beam offers symmetrical support without causing a detrimental change in the amount of beam deflection.

Claims

What is claimed is:

1. A method for controlling the vibrations of a doctor blade beam of a coater station in an assembly, wherein the doctor blade beam (1) is supported at its both ends by a pivotal joint (2) and further by at least one actuator means (3) for adjusting the position of the doctor blade beam (1), characterized in that the doctor blade beam (1) is supported with the help of at least one member (4, 5) of controllable stiffness such that changing the stiffness of the support assembly controls the natural frequency of the doctor blade beam (1).

2. The method of claim 1, characterized in that the doctor blade beam (1) is supported with the help of at least two members (4, 5) of controllable stiffness that are adapted symmetrically about the middle point of the doctor blade beam.

3. The method of claim 1 or 2, characterized in that such a support member (4, 5) of controllable stiffness is used that is lockable stiff and again releasable freely movable, such as a lockable sliding joint or a hydraulic cylinder.

4. The method of any one of foregoing claims, characterized in that such a support member of controllable force is used in which the stiffness is adjustable by, e.g., frictionally, hydraulically or pneumatically.

5. An assembly for controlling the vibrations of a doctor blade beam of a coater station, the assembly comprising a doctor blade beam (1), a pivotal joint (2) adapted at both ends of the doctor blade beam (1) for supporting the same and at least one actuator means (3) for adjusting the position of the doctor blade beam (1), characterized by at least one member (4, 5) of controllable stiffness for supporting the doctor blade beam (1) such that changing the stiffness of the support assembly controls the natural frequency of the doctor blade beam (1).

6. The assembly of claim 5, characterized by at least two members (4, 5) of controllable stiffness adapted symmetrically about the middle point of the doctor blade beam.

7. The method of claim 5 or 6, characterized in that the support member (4, 5) of controllable stiffness is lockable into a stiff position and is again releasable freely movable, such as a lockable sliding joint or a hydraulic cylinder.

8. The assembly method of any one of claims 5 - 7, characterized in that the member (4, 5) of controllable stiffness is a lockable sliding joint (6) connected to the doctor blade beam (1) by a releasable tapering pin joint.