Resilient roll for smoothing webs

Details Resilient roll for smoothing webs Resilient roll for smoothing webs

2000-05-31

2002-10-29

Cuda-Rosenbaum, I. (Department: 3726)

Roll or roller

Concentric layered annulus

Fiber or wire reinforced

C492S054000

Reexamination Certificate

active

06471626

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
The present application claims priority under 35 U.S.C. § 119 of German Patent Application No. 199 25 419.2, filed on Jun. 2, 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, e.g., for smoothing paper webs, having a hard roll core which includes, e.g., metal, and which is provided on its outer side with a resilient covering layer including a resilient matrix material, and a metallic top layer provided on the outside of the resilient covering layer. Furthermore, the invention is directed to a process for producing such a roll.
2. Background of the Invention
Resilient rolls of this type are used, for example, in the calendering of paper webs. Here, in each case an elastic roll together with a hard roll forms a press nip, through which the paper web to be processed is led. While the hard roll has a very smooth surface, made of, e.g., steel or hard cast iron, and is responsible for smoothing that side of the paper web which faces it, the resilient roll acting on the opposite side of the paper web has the effect of evening and compacting the paper web in the press nip. The rolls have lengths on the order of approximately 3 to 12 m and diameters on the order of approximately 450 to 1500 mm. They withstand line forces up to approximately 600 N/mm and compressive stresses up to approximately 130 N/mm
2
.
Since 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), higher requirements than hitherto are placed on the rolls of the smoothing device, in particular in relation to their temperature resistance. The high transport speeds of the paper web, necessitated by on-line operation, and the associated high rotational speeds of the calender roils, increase the nip frequency of the rolls, i.e., the frequency with which the cover is compressed and relieved again, which in turn leads to increased roll temperatures. These high temperatures, produced during on-line operation, lead to problems which, in the case of known resilient rolls, can lead to the destruction of the synthetic covering. On the one hand, in the case of known synthetic coverings, maximum temperature differences of about 20° C. over the width of the roll are permissible and, on the other hand, the polymers normally used for the coating have a significantly higher coefficient of thermal expansion than the steel rolls or hard cast rolls normally used, so that, as a result of an increase in the temperature, high axial stresses occur between the steel roll or hard cast roll and the synthetic coating connected to it.
As a result of these high stresses, associated with heating locations, which occur in particular at certain points, within the synthetic coating, so-called hot spots can occur, at which separation or even bursting of the synthetic layer takes place.
These hot spots occur in particular when, in addition to the mechanical stresses and the relatively high temperature, there are crystallization points in the form of, e.g., faulty adhesive bonds, deposits or above-average indentations in the resilient covering, e.g., as a result of creases in or foreign bodies on the paper web. In these cases, the temperature at these crystallization points can rise from normally approximately 80° C. to 90° C. to more than approximately 150° C., which results in the aforementioned destruction of the synthetic layer.
A roll whose resilient covering layer is provided with an additional metal coating is disclosed by DE-A-4126232. Via the metallic top layer, the thermal conductivity of the outside of the roll is increased, so that undesired heat occurring within the press nip can be dissipated rapidly, and the temperature during the treatment of the material web can essentially be kept constant.
The problem with a metallic covering layer is that the resilience of the surface of the roll decreases as a function of the thickness of the metallic covering layer. If the metallic top layer is selected to be too thick, the resilience of the surface of the roll becomes so low that the calendering result suffers under this. Furthermore, the application of an appropriately thin metal layer is relatively complicated, and it is difficult to distribute a metal layer of the desired low thickness uniformly over the entire roll surface. Furthermore, the connection between the resilient covering layer and the metallic top layer is inadequate in many cases, so that separation of the metallic top layer can occur during operation.
SUMMARY OF THE INVENTION
The present invention provides a roll of the type mentioned above which also has a good thermal conductivity at least at its surface. In this way, the metallic top layer can be simply and cost-effectively produced and can be reliably connected to the resilient covering layer underneath. In addition, a process for producing such a roll is to be specified.
According to the invention, the roll can be similar in general to the roll disclosed above that also includes a metallic top layer that includes metal fibers and/or metal-coated fibers. A corresponding process according to the invention, in order to produce the metallic top layer, includes winding a large number of metal fibers and/or of metal-coated fibers onto the resilient covering layer.
As a result of the use of metal fibers or metal-coated fibers for forming the metallic top layer, a very thin top layer can be produced and, if metal-coated fibers and a suitable fiber material are used, the top layer can have a higher resilience than a top layer made of pure metal. Via the metallic top layer, good thermal conductivity of the surface of the roll is achieved, so that the heat occurring within the press nip during operation can be dissipated rapidly to the outside. This ensures that the temperature during the treatment of the material web in the press nip can be essentially kept constant.
Furthermore, by radially inwardly projecting fibers, i.e., from the metallic top layer into the resilient covering layer, the connection between the metallic top layer and the resilient covering layer can be improved. In this manner, separation of the top layer during operation can be largely ruled out.
According to an advantageous embodiment of the invention, the fibers are combined into one or more fiber bundles and/or fiber rovings and/or fiber non-wovens. In particular, when forming the metallic top layer with metal-coated fiber rovings or those including metal, which essentially have of a large number of fibers lying beside one another in a single ply, a very thin metal layer with a high flexibility is produced.
The metallic top layer can preferably have a radial thickness between about 2 &mgr;m and 30 &mgr;m, and preferably between about 5 &mgr;m and 10 &mgr;m.
The surface of the metallic top layer is preferably ground and, in particular, polished, to create the smoothest possible roll surface for producing high-quality paper. In this case, the surface of the metal top layer can have an R
a
value less than approximately 0.05 &mgr;m, and in particular less than approximately 0.03 &mgr;m.
According to a preferred embodiment of the invention, the metal of the top layer is a metal which melts at low temperatures, e.g., at temperatures below about 600° C., preferably at about 480° C. or less, such as zirconium. As a result of using a low-melting-point metal, the production of a roll designed in accordance with the invention is simplified, since both the fiber material and the matrix material need to have a lower resistance to heat than if a metal melting at high temperatures were to be used. The use of zirconium is advantageous, since zirconium enters into a particularly good connection with the plastic materials normally used to form the resilient covering layer. As a result, the connection between the resilient covering layer

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