Calender roll and process for operating a calender roll

Details Calender roll and process for operating a calender roll Calender roll and process for operating a calender roll

2001-02-23

2002-10-15

Chin, Peter (Department: 1731)

Paper making and fiber liberation

Processes and products

With measuring, inspecting and/or testing

C100S15500G, C492S020000

Reexamination Certificate

active

06464834

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. § 119 of German Patent Application No. 100 08 800.7, filed on Feb. 25, 2000, the disclosure of which is expressly incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a calender roll having a roll jacket that surrounds an inner chamber. Furthermore, the invention relates to a process for operating the calender roll having a roll jacket that surrounds an inner chamber.
2. Discussion of Background Information
Such calender rolls are frequently used as center rolls in a calender, thus forming a part of a roll stack. Here, neighboring rolls form nips, through which a material web is guided in order to be exposed to an increased pressure or perhaps an increased temperature.
Such rolls are widely used for the processing of a paper web which in the following is used as the example for describing the invention. However, the same problems occur with other material webs as well.
In such calenders, usually so-called “soft” rolls cooperate with “hard” rolls. Sporadically, two “soft” rolls cooperate. Here, the soft rolls are covered with a plastic cover. It has been observed that after a certain duration of operation a so-called bar ring can be observed. This bar ring appearance causes, on the one hand, an undesired lined pattern on the paper web. However, these lines are also discernible as lineshaped markings on the elastic rolls with the plastic covers. The soft rolls become multi-angular, so to speak. A certain depth and intensity on these bar rings leads to rejections in the paper web. Then the correlating roll or the correlating rolls must be reworked in order to remove the deformation of the plastic cover. This is generally done by turning the roll off on the lathe.
SUMMARY OF THE INVENTION
The invention is based on the aspect of avoiding the appearance of bar rings. This aspect is attained in a process of the type described at the outset such that an active oscillation is created in the inner chamber that affects the roll jacket.
A roll stack formed from several rolls has a multitude of natural frequencies. Among these natural frequencies there are, for instance, natural sagging frequencies that are of lesser interest in the present case, and forms of natural oscillations that result from the oscillating roll masses on the spring and damper systems that are formed by the plastic covers positioned between the rolls and also by the roll bearings. An operating calender creates exciting forces whose frequencies are dependent on the roll rotations and are equivalent to a multiple of the rotation frequency in the first feeding. These exciting forces can have many causes, for instance, inhomogeneities, anisotropies, or geometric faults. Also, variations in paper thickness of the paper web traveling through the calender can excite the roll stack. When one of these exciter frequencies meets one of the natural frequencies the oscillating system reacts with increased oscillation amplitudes. Due to the multitude of possible forms of natural oscillations these resonance points are practically unavoidable in the construction.
Generally, the oscillation system is dampened to such an extent and the exciter forces are so small that the resulting oscillating motion is not immediately disturbing. Over a more or less long duration, however, these oscillating motions imprint the plastic cover of the elastic roll. Frequently it can be observed that the nearest integral multiple of the oscillation frequency is imprinted onto the roll as a pattern. Thus, a feedback of the oscillation occurs. The oscillation amplitudes then rise exponentially. They are expressed, on the one hand, by an increased sound level (up to more than 115 dB (A)) and, on the other hand, in periodical thickness variations of the paper web passing through which lead to rejections, as mentioned above.
According to the invention, the feedback effect of the creating mechanism of the periodic cover waviness is disturbed in order to prevent the above-described feedback. In order to avoid that an even integral pattern with a frequency close to the natural frequency of the system is imprinted onto the rolls, an oscillation that feeds back to the roll jacket is actively created in the inner chamber of the calender roll. In addition to the oscillations at the calender that can be observed at the roll jacket during operation, the roll jacket is caused to oscillate as well. This results in a common oscillation behavior of a single roll in which, in the ideal case, the oscillations excited from the calender are eliminated by the active oscillation created in the inner chamber of the calender roll. Frequently, this can only be achieved with difficulties in practical use. Therefore, it will frequently be sufficient to control the active oscillation such that the resulting oscillation of the calender roll does not result in a bar ring formation.
Here, it is preferred that a resulting oscillation is determined at the roll jacket and the active oscillation is controlled dependent on the resulting oscillation, i.e., an active oscillation is created in a control circuit that is designed such that the resulting oscillation is as small as possible.
Preferably, the frequency and/or the phase of the active oscillation is modified during operation. It can, for instance, be provided that the frequency and/or phase of the active oscillation is modified from time to time or continuously. To express it even more generally, the spectral parts of the active oscillation can be modified during operation with regard to their amplitudes, frequencies and/or phases. This causes the surface of the plastic cover to deform so that the impression of bar rings or the multi angularity can be avoided.
Preferably, the active oscillation is directed essentially in the pressing direction of the roll. This is the direction from which the strongest influence for the formation of bar rings is to be expected according to present knowledge. When the active oscillation of the exciter oscillation counteracts here. its impact is the largest.
This aspect is attained in a calender roll of the above-mentioned type such that at least one actuator effecting the roll jacket is provided in the inner chamber. Such an actuator can impose an oscillation onto the roll jacket. This oscillation is then the above-mentioned “active oscillation.” The oscillation caused by external exciters and the oscillation of the roll jacket caused by the actuator result then in a common oscillation that can be formed by a corresponding control of the actuator such that the formation of bar rings is avoided or at least reduced.
Here, it is preferred to position the actuator between an inertia mass and the roll jacket. This increases the effectiveness of the actuator. For example, the actuator can be embodied such that it enlarges or reduces the distance between the inertia mass and the roll jacket which requires a shifting of the inertia mass. In this case, due to the inertia of the inertia mass a reaction force develops at the roll jacket that can be used for the creation of an active oscillation.
Preferably, several actuators are distributed over the axial length of the roll jacket. This is due to the fact that many roll jackets are flexible to the extent that an impulse of an actuator positioned in the middle is not necessarily sufficient in order to make the correlating active oscillation reach to the edge strips of the roll. When several actuators are distributed in the axial direction it can be ensured that the calender roll is excited over its entire axial length. Such embodiments are certainly preferred in which all such actuators are operated in a synchronized manner or at least by the same means.
Preferably, at least two actuators, impacting perpendicular to the roll axis and being positioned essentially at the same axial position, are combined to a group. This results easily in the ability to influence the direction of the exciti

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