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To weld or not to weld the face of kiln rollers?


Is it possible to weld repair the face of a kiln roller?



The repair involves partial or complete coverage of the rolling face to build up material to replace worn off material by welding. This type of repair is frequently attempted but does not give satisfactory results in most instances. There are many factors involved which determine the success or failure of the repair. In order to understand where it may succeed and where it may fail an understanding of the mechanics of the rolling surfaces is necessary. Hertz pressure is the contact pressure between the rolling surfaces of the tire and the roller. For any given load that a roller must support, the maximum allowable Hertz pressure must not be exceeded. The diameter of the tire is mostly dictated by the diameter of the vessel it must circumvent and the thickness it must have to provide sufficient beam strength and stiffness to support the weight of the shell and its contents. The tire must therefore have an adequate section modulus. But both the thickness and the width of the tire determine the section modulus. The diameter of the roller is a determining factor for Hertz pressure. The larger the diameter of the roller the more the Hertz pressure is reduced. But the length of the line of contact between the tire and the roller (tire face width) has a more direct effect on Hertz pressure than does roller diameter. Therefore the wider the tire the lower the Hertz pressure. The line of contact between the tire and the roller, the pinch point, when viewed closely enough, is actually an area rather than a line. This is necessarily so because even steel has elasticity. Like the tire on a car, where it flattens slightly as it meets the surface of the road, so do both tire and roller surfaces flatten when they pass through the pinch point. See fig. 1. The line of contact between the tire and the roller, the pinch point, when viewed closely enough, is actually an area rather than a line. This is necessarily so because even steel has elasticity. Like the tire on a car, where it flattens slightly as it meets the surface of the road, so do both tire and roller surfaces flatten when they pass through the pinch point. See fig. 1. The force of contact begins as the curved surfaces meet. The force of contact rises to a maximum at the center of the width of contact. This would be the point on the surface intersecting the line joining the center of the tire to the center of the roller. See fig. 2. Then there is the phenomenon defined as Poisson's Ratio, which states that if material is compressed in one direction it grows in the other. See fig. 3. When the surface of the roller is welded, the new layer of metal will have different mechanical properties than that of the base material. No matter how carefully the metallurgy of the welding rod was matched to the base metal, no matter how thoroughly the work was preheated, post heated etc. there will be a difference. Often none of these precautions are taken which makes matters worse. See fig. 4. As the surface passes through the pinch point the metal compresses as shown. The depth of compression varies with geometry and total load. But the upper layer compresses differently from the underlying material because of the differences in their mechanical properties. Poisson's ratio is the measurement of this difference. Shear stresses in the transition zone can therefore easily act to separate the materials. Microscopic cracking develops and soon the weld material spalls, peels or otherwise separates itself from the base metal no matter how well the welding was done. If the roller is large enough to make repairs worth while, machining away the remaining portion of the rim is the first step to lasting repair. A new sleeve, usually a newly forged rim is then shrunk fit onto the old machined roller. This assembly is then finish machined. This is a superior type of repair. The final assembly will provide good serviceability.



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