Patent Publication Number: US-6908420-B2

Title: Dimensionally stabilized roller body

Description:
FIELD OF THE INVENTION 
   The invention relates to a roller body for a roller, for pressure-treating and/or temperature-treating a web-shaped medium, for example a paper web or thin metal web, and to a method for manufacturing said roller body. The roller body can in particular be a roller body of a calender roller of a paper machine. 
   BACKGROUND OF INVENTION 
   Reports are increasingly occurring that new calender rollers are becoming “bent” in the calender after a certain running time. These are measured central deviations of the order of 0.1 mm or a bend of about 0.05 mm. The fact that in practice such reports only come from rollers provided with a relatively thin hard coating allows the conclusion that this is not a new phenomenon, but that the incidence of bending has only come to the attention of the calender operator due to the thin coating. For, while it has previously been possible to correct such bending by routinely re-grinding—the re-grindable hard shell has had a thickness of over 8 mm—, correcting by grinding for a layer thickness of 0.1 to 0.15 mm practically always means losing the whole, expensive coating. 
   When a gray casting body—and the core of clear chill casting rollers such as the invention preferably relates to is such a body—is produced, it experiences practically no mechanical loads during the whole production, beyond the load of the weight of the body. This means that even a slight additional load, e.g. due to the swing of the roller body which is mounted at its ends during transport from the manufacturer to the operator, repeatedly triggers additional tensile stresses which cause a slight, permanent deformation of the roller body. Loads can also for example arise if the quick-release mechanism separates the rollers during operation. This is for example the case if the material web to be treated tears and a heated roller in a calender comes into direct contact with a roller comprising an elastic coating. A specific mechanism then separates the rollers and the rollers fall downwards, where they are collected by a device. This exerts acceleration forces of up to 2 g on the rollers. 
   Consider the order of magnitude: rollers are produced for example with a diameter of 812 mm and a roller length of 9,000 mm with a production tolerance of 0.005 mm with respect to the concentricity precision. In the case of a hard coating or chromium plating, a layer thickness of 0.15 mm is usually applied. A bend of 0.05 mm means a concentricity error of 0.1 mm, i.e. a deterioration in the tolerance by a factor of 20. At high rotational speeds, this incurs an imbalance and additional excursion. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to produce a roller body which no longer bends under the loads to be expected. 
   This object is solved by the subjects of the independent claims. Advantageous developments are described in the dependent claims. 

   BRIEF DESCRIPTION OF THE INVENTION 
   The invention is based on the following insight: an analysis of the causes of bending in roller bodies made of gray cast iron leads to a peculiarity of gray cast iron having a laminar formation of the dispersed graphite. As the molten iron is cooled, at temperatures of 1150 to 1300° C., the carbon dissolved in the iron is dispersed in a laminar formation. In the course of further cooling, the material shrinks, causing significantly increased stresses locally at the ends of the lamellae. Additionally superimposing a tensile stress, even at a very low tensile stress level, is enough to cause plastic deformation in these local stress centers, which causes a slight, undesired change in the form of the casting body as a whole. 
   A roller body in accordance with the invention was subjected to a load in accordance with the invention, in the course of production or afterwards, said load leading to stresses in the roller body which are larger than those which have to be expected in subsequent operation, or during transport. The load leads to plastic deformations in the peripheral regions of the graphite lamellae which are then normally not exceeded in its subsequent use. Highly pre-stressed regions around the ends of the lamellae are loaded beyond the elastic limit by means of an additional stress, such that they are plastically deformed. If the roller body is relaxed again from this additional stress, the pre-stress is reduced to the difference between the elastic stress limit and the stress additionally applied by way of the treatment in accordance with the invention. The roller body&#39;s own stress has been reduced. In order to elastically deform the roller body again, an additional stress must be applied which goes beyond the “training stress”, i.e. beyond the additional stress applied within the framework of the treatment in accordance with the invention. If the additional stress applied within the framework of the treatment in accordance with the invention has been selected high enough, then the roller body can no longer be plastically deformed during subsequent operation. As a result, selectively determined regions of the roller body are placed under an additional stress. A roller body which has already got these deformations over with during production, subsequently behaves almost completely elastic under normal loads. Such a roller body can no longer be permanently deformed, but rather elastically returns when relaxed. It is dimensionally stabilized. 
   Stabilizing in accordance with the invention is performed once the casting body has been cast and solidified. It can be performed before or after a surface treatment, for example coating or grinding, of the roller body. It can also be used to correct a plastic deformation which has already occurred, i.e. for straightening. 
   The inner stresses can be generated in various ways. The stresses are preferably applied mechanically or thermally or mechanically and thermally in combination. 
   In a preferred mechanical treatment, the roller body or roller is held fast at its ends, e.g. clamped in a lathe. By exerting a pressure, preferably directed radially to the retarded rotational axis of the roller body, onto the roller body in its axial center, the roller body is bent, and stresses are therefore generated in the roller body. This pressure is expediently applied from above, because the stresses generated in this way are added to those already generated by the roller body&#39;s own weight. If the pressure is applied from the side, then the actual load on the roller body follows from the force diagram. A load from below must first balance out twice the roller body&#39;s own weight before there is an additional load. Here, it should be taken into account that a load localized approximately in the center generates a stress which is approximately 1.6 times larger than an equivalent linear load distributed evenly over the roller barrel. 
   If the load is applied by one or more pressure bodies, e.g. by a hydraulically pressed roll, then the roller body ejected by it can be rotated while loaded. The plastic deformations at the ends of the graphite lamellae roll back asymptotically, such that the material structure or roller body is stabilized after only a few revolutions. Attention must be paid here to Herz&#39; pressing, generated by the roll in the roller material, in order that the roller body itself is not damaged. 
   Instead of continuous rotation, it is also possible to apply the load to a stationary roller body. In this treatment, the roller body is preferably rotated a little further after each load. It can, for example, be rotated by approximately 30° each time. Such a repeated, static load has the advantage that the pressure can be applied using a molded piece adapted to the roller barrel. This reduces the specific surface load. The type of treatment is also advantageous for roller bodies which already have a ground surface and/or are already coated, to retro-stabilize them. 
   If a roller body is already bent, the sag can be corrected before or after a stabilizing treatment, by redressing the roller into a position by applying a corresponding force. 
   Using the invention, a roller body which has become bent during use, i.e. through operating the roller, can advantageously be straightened mechanically by returning the roller body to a position by applying a force which redresses it into a straight shape. The redressing force can be applied as described above with respect to stabilizing. The roller body is preferably straightened in a number of steps differentiated according to the size of the redressing force. In a first step, the roller body is bent back using a preliminary force. The preliminary force is between 30 and 70%, for example 50%, of the force ultimately required to bend the bent roller body back into its straight shape. The preliminary force is then increased in stages or possibly continuously up to the force required to bend the roller body back into its straight shape. If, while being straightened, the roller body is bent in a direction perpendicular to the redressing force, the direction of the redressing force can be correspondingly changed with respect to the roller body. The extent of such undesired bending is, however, negligibly small as compared to the bending of the roller body before it is straightened. 
   The stresses in accordance with the invention can likewise be generated by generating a temperature difference. If the tensile stress in the course of such a thermal treatment reaches a level all over which goes beyond what can be expected in the roller&#39;s subsequent lifetime, then stabilizing in the sense of the invention using a heat treatment is also possible. 
   In the case of a thermal treatment, a temperature gradient is preferably generated in the radial direction in the roller body. 
   The temperature difference dt for generating a local tensile stress X is set at the location of said tensile stress such that it exhibits the value:
 
 dt=X/ ( a×E )
 
as compared to the remaining roller average, where a is the thermal expansion coefficient and E is the modulus of elasticity of the material of the roller body.
 
   The stresses are generated in such a way that they are balanced out over the sum of the roller cross-section. Tensile and compressive stresses automatically counterbalance. Preferably, all regions of the roller body&#39;s cross-section are placed under tensile stresses using a number of consecutive thermal treatments. 
   In a preferred thermal treatment, a hollow-cylindrical roller body is heated from without and within by rapidly heating it in a furnace for thermal annealing treatments comprising a strong inner blast, such that tensile stresses arise in the radial center of the roller body wall, because this region only absorbs the temperature with a time lag. By then cooling the soaked roller body by means of an inner and outer temperature drop, the stress profile is inverted. Regions experiencing compressive stresses during heating are exposed to tensile stresses during cooling. This treatment is repeated one or more times as appropriate. 
   Such temperature differences and a heat transfer fluid flowing though the roller body can also be generated by accelerated heating followed by forced cooling when operable by means of a heating and cooling facility. However, there is a danger here of destroying the roller body, in particular in the cooling phase. For, by introducing peripheral bores into the roller body, chamfers are generated in the vicinity of the center of the roller body, because the bores running towards each other never meet exactly. These chamfers, harmless in the heating operation—the region is generally experiencing compressive stresses—represent a highly sensitive point for the cooling operation. 
   A thermal treatment can also be advantageously used to correct a sag which is already present. For this purpose, the roller body is preferably heated or cooled, or also heated and cooled in combination, on one side only. In this way, stresses can be specifically generated in a desired direction. Lastly, reference is made to be fact that, as already mentioned with respect to stabilizing, a thermal treatment can also be used in combination with a mechanical treatment for the purpose of straightening. 
   Straightening in accordance with the invention, whether purely mechanical, purely thermal, or thermomechanical, is likewise preferably performed in such a way that plastic deformations of the roller body in its subsequently to be resumed operation are counteracted in the same sense as described with respect to stabilizing in accordance with the invention. 
   The mechanical method exhibits advantages as compared to the thermal treatment. The desired effect is achieved more rapidly and at lower cost. Moreover, the type of load corresponds precisely to that which also leads to the described dimensional defects in the roller body&#39;s subsequent lifetime. In subsequent roller body operation, thermal loads occur almost exclusively symmetrically, because the roller is then rotating.