Resilient roll and process for producing such a roll

Details Resilient roll and process for producing such a roll Resilient roll and process for producing such a roll

2000-03-30

2001-11-20

Vo, Peter (Department: 3721)

Manufacturing container or tube from paper; or other manufacturi

Surface treatment

C493S141000, C493S144000, C493S145000

Reexamination Certificate

active

06319185

ABSTRACT:

CROSS-REFERENCE RELATED APPLICATIONS
The present application claims priority tinder 35 U.S.C. § 119 of German Patent Application No. 199 14 710.8, filed on Mar. 31, 1999, 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 roll of the type used for smoothing paper webs. The roll has a hard roll core which can be a metal and an outside surface utilizing a resilient covering layer. The covering layer may be a resilient matrix material with fibers embedded in the matrix material. Furthermore, the invention is directed to a process for producing such a roll.
2. Discussion of Background Information
Resilient rolls of this type are typically used, for example, in the calendering of paper webs. Such calenders often use an elastic roll together with a hard roll in forming a press nip. The paper web is calendered by feeding it through one or more of these nips. The hard rolls generally have a very smooth surface and are made of, for example, steel or hard cast iron. They function in smoothing that side of the paper web which faces it. Resilient rolls which act on the opposite side of the paper web have the effect of evening and compacting the paper web in the press nip. The resilience of this second or opposite roll in the nip acts to limit intensive compaction of the paper web, which would lead to a specky appearance of the paper web. Such rolls are generally large and typically have lengths of from about 6 to 12 meters and diameters from about 800 to 1500 mm. Moreover, they are designed to withstand line forces up to approximately 600 N/mm and compressive stresses up to about 50 N/mm
2
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The tendency in paper manufacture is for calendering to be carried out on-line, that is to say the paper web leaving the papermaking machine or coating machine is led immediately through the paper smoothing device (calender). This design places high requirements or demands on the rolls of the calender or smoothing device. In particular, this design subjects the rolls to higher temperatures so that they are require to have temperature resistance. The high transport speeds of the paper web, necessitated by on-line operation, and the associated high rotational speeds of the calender rolls increase the alternating flexure frequency of the rolls. It is these factors which in turn leads to increased roll temperatures.
These high temperatures which are produced in on-line operation lead to problems which, in the case of conventional resilient rolls, can lead to the destruction of the synthetic covering. Such conventional synthetic coverings can function only with a maximum temperature differences of about 20° C. over the width of the roll. Moreover, the polymers normally used for the roll coating have a significantly higher coefficient of thermal expansion than the steel rolls or hard cast rolls normally used. Thus, when there is an increase in the temperature of the rolls, high axial stresses occur between the steel roll or hard cast roll and the synthetic coating which is connected to it.
Moreover, such rolls also experience high stresses in localized regions of the roll due to these regions being heated more so than surrounding areas. Such hot spots in the synthetic coating can cause the synthetic layer to separate or burst from the metal roll.
These hot spots can occur when, in addition to the mechanical stresses and the relatively high temperatures experienced by the rolls, there are crystallization points in the form, for example, of faulty adhesive bonds between the layer and the metal. Additionally, deposits or above-average bulges in the resilient covering which result from creases or foreign bodies on the paper web can produce these hot spots or crystallization points. In these cases, the temperature of these crystallization points often rises from normally about 80° C. to 90° C. to more than 150° C., which results in the aforementioned destruction of the synthetic layer.
SUMMARY OF THE INVENTION
The present invention provides a process for producing a resilient roll of the type mentioned at the beginning. Moreover, the invention is also directed to a corresponding roll. The roll of the invention is designed to withstand the formation or occurrence hot spots, with at least constant mechanical characteristics.
According to the invention, the roll utilizes a varied fiber content of the covering layer. The fiber content of the layer varies radially from the inside to the outside, and in particular, the fiber content is decreased from the inside to the outside of the layer. A corresponding process according to the invention provides for varying the fiber content of the covering layer in the radial direction, and in particular making the roll with a radially variable reduced fiber content so that the fiber content decreases from the inside to the outside.
Varying the fiber content of the covering layer radially from the inside to the outside results in a covering layer which has a coefficient of thermal expansion which, corresponding to the fiber content, differs in a radial direction from the inside to the outside. Since the matrix material usually has a considerably higher coefficient of thermal expansion than the fiber material used, the respective resulting coefficient of thermal expansion of the matrix material to which fibers have been added depends both on the coefficient of thermal expansion of the matrix material, to which fibers have been added, and that of the fibers. The more fibers which are embedded in the matrix material, the more similar the resulting coefficient of thermal expansion is to the coefficient of thermal expansion of the fibers used.
In this way, it is possible to adjust the coefficient of thermal expansion of the radially inner region of the covering layer by utilizing a relatively high fiber content. Thus, this inner region of the covering layer can be made so that it has the same order of magnitude coefficient of thermal expansion as that of the roll core. Accordingly, when the roll experiences heating from operation, the radially inner regions of the covering layer thus expand by essentially the same value as the roll core, so that high axial longitudinal stresses between the roll core and the covering layer are avoided or reduced significantly.
Since high fiber content also increases the stiffness of the covering layer considerably, the fiber content must be selected so that it is lower in the radially outer regions of the covering layer. Otherwise the surface of the roll may be too hard and would not be suitable for calendering. A fiber content which, in particular, decreases essentially continuously radially outwards within the covering layer provides the advantages of good calendering and a longer lasting roll. Thus, during roll heating, the longitudinal stresses which typically occur within the covering layer and which are produced because of the different thermal expansion of the various regions of the covering layer, do not become so great at any point that detachment or destruction of the covering layer arises.
According to one embodiment of the invention, a roll is provided in which, in the radially outer region of the covering layer, the fiber content is essentially zero. This means that the surface of the roll is as resilient as possible and, following an appropriate grinding operation, has a very smooth surface. In this embodiment, all of the fibers present in the covering layer are disposed within the covering layer, but do not reach as far as the surface of the covering layer. This design allows the surface of the roll to be reground after a certain running time without the fibers present in the matrix material emerging from the covering layer after the grinding operation.
The nip may also be adjusted to operate efficiently with the desired fiber content of the roll cover. In the case of a roll having a very resilient cover, the hard mating roll may be impressed more intensely into the soft covering of the elastic roll. This can resul

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