| Roll milling is the main machine of wheat flour processing, the design of associated equipment and manufacturing has a century of history. Roller mill is widely used in wheat flour processing because of its high output, easy operation, short grinding time, low temperature rise and good quality of flour. However, as technology advances, flour products is required in higher quality, so it is especially important that study the mechanism of roller and explore the new methods to improve work efficiency and milling quality. In this paper, the system of the milling process, milling equipment and roller status were summarized and the development, structure and operating parameters were studied, the flow field, failure modes and mechanism of milling and wear of the metal milling rollers were analyzed. The main contents are as follows:1. The basic structure of the roller, the parameters of structure and operating and roller failure modes were analyzed in detail. The results showed that the roller is suffered a larger radial pressure and a strong friction in processing, therefore the milling rollers should not only have high strength, toughness, wear resistance and good thermal conductivity, but also not have any manufacturing defects of air bell, pores, cracks and hardness uneven on roller surface. It also showed that the main milling roller fiction was wear failure which lead to surface roughness of the roller gradually decreased and uneven of work areas along the length direction, thereby reducing the abrasive effect.2. By wheat compression mechanical tests, the mechanical parameters of single wheat were obtained by compression load and found that the compressed form, the moisture content have a significantly effect on the mechanical parameters. By using crushing theory of a single particle, the mechanical model of roller-single wheat-roller was established while the rolling angle and force were calculated. By using crushed theory of the material layer, a mechanical model of the roller-wheat powder-roller was established while a positive pressure, shear force and friction force were calculated. The results of finite element analysis showed that, roll milling had a uniform axial stress distribution, the maximum radial stress was at π/2central angle and3-5mm away from roller surface.3. Regarded the wheat powder as a granular flow, the CFD methods was used to establish the mathematical model of material particles stream. The simulated analysis of particle was made by the FLUENT software. The curve of residual plots had good convergence. The visual pressure distribution was obtained which include fast roll speed of300r/min, the speed ratio of1.25for the best working condition. 4. Mill roll wear is a three-body abrasion. From principles of Tribology, the wear mathematical model was established. The wear tests of common metallic roller were designed by orthogonal method. The results showed that, the abrasive particle size, speed and roller span had different influence on white iron. The impact of the speed was extremely significant which followed by abrasive particle size. Worn morphology analysis showed that the wear of white iron and gray iron had the fatigue off, cutting and breaking. The differences of hardness, toughness and organizational lead to different wear form.During the process, the chemical reaction of material and roller lead to the metal element content of surface decreased, while non-metallic elements were increased. Because of higher hardness of white iron carbide, which could not easily be cut, the white iron has a good abrasion resistance while the gray iron has poor abrasion resistance.The analysis methods and conclusions in this study will provide the theoretical basis for actual engineering design of the roller. The wear test results have a great practical effect on prolonging the roller replacement cycle and reduce the magnetic metal content in the flour, which has important theoretical significance and value of engineering applications. |
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