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

2000-05-31

2003-02-18

Rosenbaum, I Cuda (Department: 3726)

Roll or roller

Concentric layered annulus

Specific composition

C492S056000, C492S058000, C492S059000

Reexamination Certificate

active

06520897

ABSTRACT:

The present application claims priority under 35 U.S.C. §119 of German Patent Application No. 199 25 418.4, 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 fillers embedded in the matrix material. 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.
In order to control the characteristics of the resilient covering layer, powdered fillers and/or fibers are embedded in the matrix material. The physical characteristics of the resilient covering layer are dominated or influenced by the quantity and physical characteristics of these fillers or the fibers.
SUMMARY OF THE INVENTION
The present invention provides a process for producing a resilient roll of the type mentioned above, as well as the produced resilient roll, in which the risk of the occurrence of hot spots is reduced.
According to the invention, the roll is similar in general to the above-described roll and also has a matrix material that includes a soft metal. A corresponding process according to the invention includes applying a soft metal to the roll core to produce the covering layer.
As opposed to the covering layers of known smoothing rolls of calendars, the covering layer of the instant invention includes a soft metal, such as zirconium, not plastic. Because of the high thermal conductivity of metal, a roll constructed in accordance with the invention can exhibit a very rapid dissipation of heat from overheating occurring within the covering layer, so that the occurrence of hot spots and the associated destruction of the covering layer are reliably prevented.
Furthermore, because both the roll core and the covering layer are produced from metal and thus have essentially a same coefficient of thermal expansion, no longitudinal stresses or no significant longitudinal stresses occur between the roll core and the metallic covering layer. In this manner, the occurrence of hot spots and separation or bursting of the covering layer is avoided.
The necessary resilience of the covering layer is ensured by the suitable selection of a correspondingly soft metal. It is advantageous if, in addition, the fillers embedded in the metal forming the matrix material have a higher resilience than the metal. In this manner, the resilience of the covering layer, which is composed of the respective resilience of the metal and fillers, is increased. In contrast to the known rolls, the fillers incorporated in the matrix material are not provided to increase stiffness of the covering layer, but, rather, to reduce it. Since, according to the present invention, the heat occurring within the covering layer during operation thus dissipated directly via the matrix material and not via the fillers, and since the matrix material has an uninterrupted thermal conductivity over the entire extent of the covering layer, the heat from undesired overheating points can be dissipated rapidly and reliably, both radially to the roll core and axially.
According to a further advantageous embodiment of the invention, the metal forming the matrix material is a metal which melts at low temperatures, e.g., at temperatures below about 600° C., and preferably at about 480° C. or less. The use of a low-melting-point metal simplifies the production of a roll constructed in accordance with the invention, since the material of the roll core and the material of the fillers to be incorporated in the matrix material must have a lower heat resistance than that which would be required if a metal melting at high temperatures were used.
The fillers can preferably include aramide or the like and/or be formed as synthetic particles, e.g., as fibers or rovings. In the case of being formed as fibers, at least some, e.g., a predominant proportion, of the fibers can be aligned in the axial and/or radial direction or distributed randomly. In this regard, the stiffness or resilience of the covering layer can be adjusted depending upon the alignment of the fibers.
The fibers can be arranged in one or more fibre layers, e.g., arranged concentrically, and it is also possible for further fillers, in particular in the form of fibers or powder, to be arranged in the metal forming the matrix material, in order to exert an influence on the physical characteristics of the covering layer in a desired way.
In order to produce a roll constructed in accordance with the invention, the roll core can be coated with the metal. For example, the metal can be applied (evaporated on, sprayed on or the like) to the roll core in the liquid state. It is also possible, in order to produce the covering layer, for at least one fibre bundle having of a large number of metal fibers or metal-coated fibers to be wound onto the roll core, e.g., a number of fibre layers over one another. In this case, the metal fibers or the metal coating of the

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