Abstract: In order to guide the determination of the key parameters of the screw conveyor of the horizontal screw centrifuge and the optimization of the structure, a parametric three-dimensional finite element model of the screw conveyor was established by using the finite element analysis software ANSYS. The structural strength analysis and rotor modal analysis were carried out. The auger is a rigid rotor, the maximum stress appears at the root of the cylindrical section, and the maximum radial displacement occurs at the outer edge of the spiral section of the cylindrical section. The effects of blade thickness and half cone angle on the strength and inherent characteristics of the rotor structure are further analyzed. The results of parametric analysis show that the stress intensity and radial displacement increase with the decrease of the wall thickness of the spiral blade. A larger half cone angle will result in an increase in the overall stress intensity value. Key words: screw conveyor; parameterization; structural strength; optimization design CLC number: TH237 Document code: A Article ID: 1001-2354 (2010) 05-0067-04 Horizontal spiral discharge sedimentation centrifuge (referred to as horizontal screw centrifuge) is a machine for separating suspension by centrifugal sedimentation. It has been widely used in petroleum, chemical, metallurgical and other industrial sectors and sewage treatment projects. The screw conveyor is a key component of the decanter centrifuge, and its performance directly determines the separation efficiency, production capacity and service life of the centrifuge. With the continuous improvement of the production capacity and separation factor of the horizontal screw centrifuge, the inherent characteristics and strength of the large-scale, high-speed screw conveyor are directly related to the safety and reliability of the conveyor, which has become a key issue in the design [1]. Among them, the force analysis of the screw conveyor is the basis of the strength analysis. AW Roborts et al. analyzed the influence of the vortex flow of the suspension on the force of the screw conveyor [2]; Wallace W-F Leung et al. studied sludge dewatering. The calculation method of the torsional moment of the screw conveyor of the decanter centrifuge [3-4]. Based on the calculation of the force measurement and analysis method of the conveyor, N Corner-Walker of Alpha-Rafa, Sweden, studied the influence of the torsion force of the screw conveyor on the moisture content of the solid phase centrifugal product. ]. By referring to the above-mentioned series of foreign research methods and achievements, China has also begun to study the reliability of large-scale high-speed screw centrifuge rotors. Gu Wei and Qian Caifu have established a three-dimensional finite element model of the screw conveyor. The structure of the component was simulated, and the model was analyzed by static analysis and modal analysis [6]; Liu Aijun et al. applied Nastran software to check the structural strength of a certain type of horizontal screw centrifuge screw conveyor [7]. . In order to further optimize the structure of the screw conveyor, the three-dimensional parametric model of the screw conveyor was established by using the finite element analysis software ANSYS. The parametric finite element analysis of the important parameters affecting the strength and inherent characteristics of the rotor structure was carried out. The design and improvement of the device provides a reference basis. 1 Calculation model and method 1. 1 physical model The screw conveyor model used in this paper is a cylindrical conical double-headed spiral structure. The main parameters are shown in Table 1, and the structure is shown in Figure 1. For the asymmetric spiral structure, in order to realize the parametric control of modeling, the modeling time in the parametric analysis process is saved, and the 3D solid modeling of the screw conveyor is carried out by using the APDL parametric language, and the solid 45 unit is used for mesh division. The model grid structure is shown in Figure 2. The number of nodes is 18 787 and the number of finite units is 55 295. 1. 3 Load and boundary conditions [8] The loads that the auger bears when working stably include: (1) Centrifugal force generated by high-speed rotation. The centrifugal force load is applied in the form of angular velocity and can be described as: For ease of description, the centrifugal force load is recorded as T1, and the positive pressure and friction load are recorded as T2. Applying the superposition principle of the linear elasticity problem, the above load combination is finally applied to the model for analysis and calculation, and is recorded as T1+T2. According to the design structure of the horizontal screw centrifuge, the two ends of the screw conveyor are respectively supported by the inner cavity of the end caps on both sides of the drum through bearings. Therefore, one end of the solid model constrains all displacements (ALL DOF) and the other end constrains displacements in the X and Y directions. The model constraint application is shown in Figure 1. 2 Analysis of calculation results 2.1 Structural strength analysis The main purpose of the strength analysis is to examine the stress intensity and radial deformation of the screw conveyor. The reference standard for the maximum radial deformation is the clearance between the spiral blade and the inner wall of the drum, which is 1 mm. It can be seen from the radial displacement and stress cloud diagram that the radial displacement and stress distribution are relatively regular. The radial displacement value and the stress value show a significant increase trend from the direction of the spiral conveyor cone to the cylindrical section. Appears on the outer edge of the spiral section of the cylindrical section, and the maximum stress appears at the root of the cylindrical section. In the design, the gap between the straight blade and the inner wall of the drum should be properly selected in combination with the radial displacement of the spiral blade and the inner wall of the drum during the operation of the centrifuge. In the manufacturing process of the screw conveyor, appropriately increasing the height of the solder fillet at the root of the spiral blade can effectively reduce the stress at the place and prevent the occurrence of blade breakage accidents. For this analytical model, the maximum stress is 109 MPa; the maximum radial displacement is 0.033 5 mm, which is significantly smaller than the minimum clearance between the spiral blade and the inner wall of the drum. The structure meets the strength requirements and has a large margin. 2.2 rotor modal analysis The sinusoidal extraction of the auger rotor is carried out by the Lanczos method. For the model analyzed, the first 3 natural frequencies and the corresponding critical speed values ​​are shown in Table 2. The radial deformation and axial deformation cloud diagram of the auger at the first-order critical speed is shown in Figs. 5 and 6. At present, the rotational speed of the decanter centrifuge is generally lower than 10 000 r/min. From the modal analysis results, the first-order critical speed of the auger corresponding to the structural parameter is 27 114 r/min, which is much higher than that of the screw conveyor. Actual speed. Therefore, the auger does not resonate during normal operation. 3 Parametric analysis of structural strength 3.1 Influence of blade thickness h on structural strength In order to guide the improvement of blade thickness selection during the design process, the strength of the auger under the condition of blade thickness of 6~10mm is respectively analyzed. Table 3 and Figure 7 show the strength and inherent characteristics of the five structures. It can be seen from Table 3 and Figure 7 that the change of blade thickness only affects the stress generated by the positive pressure and the friction load, the stress generated by the centrifugal force load is almost constant, and the maximum stress intensity of the screw conveyor is under three kinds of load conditions. Both increase as the thickness of the spiral blade decreases. When the blade thickness is 6 mm, the maximum stress value is 198 MPa, which is close to the allowable stress value of the steel. The stress cloud diagram under various operating conditions shows that the maximum stress intensity appears at the root of the cylindrical segment. The above position is prone to fracture or plastic deformation. In the structural design, measures such as increasing the size of the weld bead at the root of the blade or improving the quality of the weld should be taken to avoid stress concentration. 5〜 2毫米。 The gap between the spiral blade and the drum is generally 0. 5 ~ 2 mm. Excessive radial displacement of the auger may cause interference between the two, which in turn causes wear and damage. Therefore, the radial displacement of the auger at different blade thicknesses is further analyzed and analyzed, and the reference to the wall thickness of the blade is provided from another angle. Table 4 and Figure 8 show the deformation and intrinsic properties of the five structures. As can be seen from Table 4 and Figure 8, as the thickness of the blade is reduced, the radial displacement of the auger gradually increases. The radial displacement cloud diagram shows that the maximum displacement occurs at the outer edge of the spiral blade of the cylindrical section, where interference is easy to occur. In the actual design, the gap between the two should be reasonably set in combination with the radial displacement of the drum. 3. 2 Effect of half cone angle α on stress intensity The semi-cone angle is one of the most critical parameters in the design of the screw conveyor. On the basis of maintaining other original structural parameters, the influence of the change of the half cone angle on the strength of the screw conveyor is analyzed. Table 5 and Figure 9 show the variation of the auger stress intensity with the half cone angle. It can be seen from Table 5 and Figure 9 that the change in the half cone angle only affects the stress generated by the positive pressure and the friction load, and does not affect the magnitude of the stress generated by the centrifugal force load. Under the condition of maintaining other structural parameters, the stress intensity increases with the increase of the half cone angle. When designing the screw conveyor, increasing the half cone angle can increase the effective settlement area of ​​the centrifuge, thereby increasing the production capacity, but it should be noted that increasing the half cone angle also increases the stress intensity value of the screw conveyor. Therefore, the half cone angle should be reasonably set within the range permitted by the strength of the screw conveyor. 4 Conclusion The ANSYS software was used to analyze the structural strength of the screw conveyor of the decanter centrifuge. The conclusions are as follows: (1) Applying APDL parametric language to realize the whole parameterized control from solid modeling, partitioning unit, applying constraints and load, and solving analysis, it is of great significance to save modeling time and improve analysis efficiency. (2) The stress and displacement cloud diagram shows that the maximum stress intensity of the auger is often found in the blade root of the cylindrical section, and the maximum displacement occurs at the outer edge of the spiral blade of the cylindrical section, which should be combined with the inner wall of the drum during the design process. The radial displacement is reasonably selected for the gap between the two. (3) The results of parametric analysis show that the stress intensity and radial displacement increase with the decrease of the wall thickness of the spiral blade. With the increase of the half cone angle, the overall stress intensity increases. In the design, the blade thickness and the half cone angle should be reasonably selected to ensure the structural strength of the screw conveyor.
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1. 2 finite element model
(2) The positive pressure exerted by the solid phase sediment on the spiral blade. Based on the principle of force and reaction force, the positive pressure of the blade can be obtained by analyzing the force of the sediment.
The normal reaction force of the blade to the sediment at the diameter of the cone section D = 370 mm (average diameter of the cone section) is:
Where: μ———the friction factor between the material and the spiral blade.
3 and 4 are respectively a radial deformation cloud diagram and an equivalent stress cloud diagram of the auger.
