2021-05-28 11:22:03

The solid model and virtual prototype model of the screw conveyor of horizontal screw centrifuge are established by using Pro / engineer system Under the environment of visual NASTRAN, the kinematics analysis, modal analysis and static analysis are carried out by using the finite element method, and the mechanical state of the bearings at both ends of the screw conveyor, the multi-order natural vibration frequency, vibration mode, stress, strain, displacement and other parameter values of the system are obtained. According to the simulation results, the strength and stiffness are checked, and the main failure forms and dangerous positions of the screw conveyor and the influence of the change of blade wall thickness on the strength and stiffness of the screw conveyor are analyzed. The results show that the existing structure basically meets the design requirements, and the optimization scheme of blade wall thickness is given, which provides a valuable theoretical reference for the structural design of horizontal screw centrifuge.

Key words: screw conveyor; Structural analysis; Virtual prototype; Finite element method; Blade wall thickness

With the rapid development of national economy and the strengthening of environmental awareness, the requirements for the production capacity, separation effect, energy saving and consumption reduction of horizontal screw centrifuge are higher and higher. How to improve the product design level, shorten the development cycle of new products and accelerate the renewal of existing products has become an urgent task. Obviously, following the traditional design means [1] can no longer meet the needs of the new situation. We must adopt advanced technologies such as optimal design, static and dynamic analysis of finite element method, computer-aided drawing and so on [2].

Screw conveyor is the main component of horizontal screw centrifuge. Based on the static analysis of screw conveyor, further parameter analysis and optimization design is of great engineering practical significance for horizontal screw centrifuge. Virtual prototyping (VP) technology is applied to simulate the static and dynamic performance of the screw conveyor. The parametric solid modeling of the product is carried out on the platform of Pro / engineer software. The virtual prototype of the screw conveyor is established through reasonable simplification, and the finite element method is applied in the finite element simulation software MSC The kinematics simulation, modal simulation, stress and deformation simulation of the screw conveyor are carried out in visualnastran to obtain the multi-order natural vibration frequency, vibration mode and the changes of stress, strain and displacement of the screw conveyor, so as to check the strength and stiffness of the screw conveyor structure, and further analyze the main failure forms and dangerous positions of the screw conveyor, According to the analysis results, the optimization scheme of the structural design of the screw conveyor is put forward.

1 load and boundary conditions

1.1 load

In the working process, the screw conveyor mainly bears the following three kinds of loads: (1) centrifugal force caused by its high-speed rotation; (2) The positive pressure applied by the separated medium (sediment) to the helical blade; (3) The friction force exerted by the separated medium (sediment) on the helical blade. Strictly speaking, the screw conveyor also bears the gravity of the equipment itself, but because the horizontal screw centrifuge has a high separation factor, that is, the gravity of the screw conveyor is far less than the centrifugal force caused by high-speed rotation, the influence of gravity is ignored in the process of calculation and simulation.

1.2 boundary conditions

Since both ends of the inner cylinder of the screw conveyor are respectively connected with the left and right journals, the journals are supported on the bearings in the inner cavity of the left and right end covers of the drum. Therefore, according to the actual working conditions, the model imposes rotational pair constraints on the left and right supports.

The strength check of screw conveyor is carried out with reference to the analysis and design method of pressure vessel [3]. The material of the screw conveyor is an alloy material, and the basic allowable stress SM is 203mpa. Since the minimum clearance between the screw conveyor and the drum is 4mm (bilateral), 2mm is used as the basis for investigating the radial deformation of the screw conveyor in this analysis.

2 virtual prototype modeling and dynamic simulation

Virtual prototype is to use advanced CAD modeling technology to establish a digital three-dimensional model consistent with the physical prototype, and then use the virtual prototype model for dynamic simulation analysis. According to the actual structure of the existing screw conveyor, the three-dimensional parametric technology and Pro / Engineer [4] software are applied to establish the three-dimensional model of each component and bearing in the drum and screw conveyor, which is then simplified into a whole, and the boss, chamfer, bolt hole and other characteristics of the model are processed without considering the welding characteristics of the weld [5]. In this way, unnecessary constraints can be eliminated, too large and complex VP model of the whole machine can be avoided, and the model can be imported into the finite element simulation software MSC through the general data exchange standard step In visualnastran [6], as shown in Figure 1, the virtual prototype model defines the material

Relevant material performance parameters such as elastic modulus, Poisson's ratio and density, mechanical parameters such as rotational speed and positive pressure, motion pair and boundary conditions are defined, and motion simulation is carried out to obtain the motion track of the screw conveyor and the stress condition of the motion pair. Among them, the force analysis results of the left and right shafts are shown in Figure 2.

3 modal analysis

This study mainly analyzes the free mode of the screw conveyor assembly. The main material characteristic parameters are consistent with the previous simulation, in which the damping coefficient is 0.01 and the total mass is 409.0kg.

Because the rotor of the horizontal screw centrifuge is a rigid shaft design, that is, the rotor speed is lower than the first critical speed, so in practice, we are more concerned about the first three natural frequencies and vibration modes. Considering its own gravity and boundary constraints, the spiral conveyor adopts tetrahedral elements to divide the grid. Its finite element model has 75430 nodes and 38451 elements. Among them, the first three order stress and displacement vibration modes are shown in Fig. 3 and Fig. 4. Main results of modal analysis (see Table 1).

It can be seen from the simulation that when the natural frequency of the first-order screw shaft deformation of the conveyor is greater than that of the first-order screw shaft deformation (28R / min), it also corresponds to the critical frequency of the first-order screw shaft deformation, that is, when the natural frequency of the first-order screw shaft deformation is greater than that of the first-order screw shaft deformation (28R / min). Since the design speed of the screw conveyor is 2742 R / min, which is far less than the critical speed, the screw conveyor will not resonate within the normal speed range.

4 strength and stiffness analysis

The static analysis of the screw conveyor under no-load and full load conditions is carried out. The load under full load condition is a linear combination of positive pressure, friction and centrifugal force. This is the most concerned problem in all working condition analysis.

Fig. 5 and Fig. 6 are the stress and radial deformation nephogram of screw conveyor under full load condition respectively. Under full load condition, the maximum stress appears at the beginning of the spiral blade at the big end of the screw conveyor. The maximum value is 152.5mpa. Under the full load condition, the maximum radial displacement of the screw conveyor occurs on the pushing surface of the radial edge of the spiral blade at the transition section across the inner cylinder of the column cone. This is the dangerous position for the radial deformation of the blade, and the maximum radial deformation is 0.09239mm. The stress distribution nephogram shows that the stress value of the pushing surface of the blade root at the large and small ends and the pushing surface of the blade root at the transition section is large, which is a dangerous position for plastic deformation or failure.

5 structural optimization

The spiral blade is in direct contact with the sediment in the horizontal screw centrifuge and plays the role of conveying the sediment. The wall thickness of the blade does not play a role in the process requirements, but through the static simulation analysis of the screw conveyor, it is found that the maximum stress appears on the screw blade, and the screw blade is prone to serious axial deformation and damage when subjected to excessive material reaction. Therefore, this analysis should not only observe the influence of the change of blade wall thickness on stress and radial displacement, but also find out the relationship between blade wall thickness and the maximum axial deformation of screw conveyor. The simulation analysis shows that the current designed blade wall thickness has safety margin. In this analysis, the blade wall thickness is reduced by 0.5mm in turn. The calculation results of stress, radial displacement and axial displacement are shown in Table 2 ~ 4.

It can be seen from table 2 ~ 4 that the maximum stress increases with the decrease of blade wall thickness under full load condition. When the blade wall thickness is 8mm, the maximum stress is greater than the basic allowable stress sm of the material, which does not meet the strength requirements; The maximum radial displacement increases with the decrease of blade wall thickness. But within the scope of permission; When the blade wall thickness is reduced, the maximum axial displacement of the blade caused by the positive pressure of sediment on the helical blade increases rapidly. When the blade wall thickness is 8mm, the maximum axial displacement has exceeded 1mm, and the blade deformation is obvious. The axial displacement generated by the centrifugal force of the screw conveyor's own mass on the blade also increases with the decrease of the blade wall thickness, but the increase speed is slow, because the centrifugal force is the main factor causing the radial deformation of the screw conveyor. Under full load condition, the maximum axial displacement under various blade wall thickness occurs on the pushing surface of the radial edge of the helical blade in the column section.

The analysis does not consider the material blockage that may occur in the actual production. The production practice has proved that once the material blockage occurs during the operation of the horizontal screw centrifuge, the blade is easy to be damaged and affect the normal production process. Therefore, the spiral blade cannot be too thin, and it also puts forward higher requirements for the treatment process of blade surface (wear resistance and surface roughness) [7]. Therefore, the blade wall thickness shall not be less than 8mm.

6 Conclusion

The dynamic simulation, modal and stress-strain analysis of screw conveyor are carried out by using virtual prototype technology. According to the simulation results, the influence of the change of blade wall thickness on the strength and stiffness of the screw conveyor is analyzed.

Main conclusions:

(1) The modal simulation of the screw conveyor shows that the transverse vibration occurs at the first natural vibration frequency, the maximum stress appears at the corresponding part of the bearing center, and the maximum displacement appears at the bearing positions and shoulders at both ends; Since the actual speed is far less than the critical speed, the screw conveyor will not resonate within the normal speed range.

(2) The static finite element simulation of the screw conveyor shows that the pushing surface near the root of the small end blade in the cone section and the pushing surface at the root of the straight end blade are the positions where the maximum stress occurs; The pushing surface across the radial edge of the spiral blade on the transition section of the cylindrical cone inner cylinder is the position where the maximum radial displacement occurs.

(3) The maximum stress increases with the decrease of blade wall thickness under full load condition; The maximum radial displacement increases with the decrease of blade wall thickness; With the decrease of blade wall thickness, the maximum axial displacement of the blade caused by the positive pressure of sediment on the helical blade increases rapidly.

These conclusions have theoretical guiding significance for the structural optimization of horizontal screw centrifuge to screw conveyor, and also show the strong advantages and development potential of virtual prototype finite element simulation.

Sui Yunkang, Du Jiazheng, Peng Xirong MSC. Nastran finite element dynamic analysis and optimization design practical course [M] Beijing: Science Press, 2004