Views: 185 Author: Site Editor Publish Time: 2023-08-15 Origin: Site
The frame is the key component of the bending machine. The rigidity of the frame directly affects the safety performance and bending accuracy of the machine. How to balance the quality and cost has always been the direction of the designer. The A series bending machine is a model that the company introduced and promoted advanced technology from abroad in the early 1980s. The series of bending machines are simple, practical and have low failure rate. They are deeply loved by users and have always been the company's hot products. Since the machine was designed before the 1980s, it was limited to the design system and computer software and hardware level at that time. At that time, the design was basically based on the traditional conventional material mechanics method. For the large-scale welded structural parts of the bending machine frame. The stress concentration point cannot be accurately calculated, and the approximate hypothesis method is often used, and the calculation result is very rough. In order to insure, designers often add artificial experience values, which increases the safety factor, resulting in equipment that is very cumbersome, which consumes materials and increases the difficulty of production.
2.1 Machine structure
The A series bending machine is the upper transmission structure, as shown in Figure 1. Mainly composed of the following parts:
Figure 1——A series bending machine
Rack: welded by thick steel plate, mainly composed of top beam, left and right side plates and bottom beams, used for fixing various components such as oil cylinder, guide rail and lower die.
Slider: The overall thick steel plate structure is connected with the oil cylinder and the guide rail, and the lower end is fixed with the upper mold, and the working cylinder drives the upper and lower reciprocating motion to complete the sheet bending.
Cylinder: Provides the bending force required to bend the sheet and drives the slider to move up and down.
Balance bar: Ensure that the slider runs synchronously to the left and right.
Slideway: fixed on the frame to limit the movement of the slider.
2.2 Research object
The A series of bending machines currently produced by the company have various specifications. This paper selects the best-selling and representative A3.1m×1000kN bending machine for research and analysis. The research object is the frame body with the most materials. Figure 2 is a three-dimensional modeling diagram of the frame of the A series bending machine. It is welded by thick steel plate and divided into three parts: the top beam, the left and right side plates and the bottom beam. The top beam is a double plate structure for mounting the drive. The oil cylinder; the bottom beam is a whole thick steel plate structure for receiving the load force of the lower mold; the side plate is used for connecting the top beam and the bottom beam, and the side plate is provided with a C-shaped throat for the purpose of feeding.
Figure 2——Rack 3D model
The frame of the bending machine is welded. If the welding structure is used during modeling, factors such as the type of weld between the steel plates should be considered, which will greatly increase the complexity of the calculation process. In order to facilitate grid generation and control, the model is guaranteed. The geometry and mechanical properties are similar to the real situation, and the following simplifications are made:
(1) Pattern generation of a single part for the rack model;
(2) In order to get closer to the actual welding situation, all welds are chamfered;
(3) Eliminate fine structures such as process holes, threaded holes, and ribs that have less influence on strength and stiffness.
3.1 Mechanical properties of materials
The racks are all welded by Q235 steel plate. The mechanical parameters of Q235 steel plate are as follows:
Elastic modulus E=210GPa;
Poisson's ratio μ = 0.28;
Density ρ = 7.8 × kg / m3;
Yield strength σs = 235MPa;
Allowable stress [σ] = 160 MPa.
3.2 Rack load and constraint description
The load of the bending machine in the actual work is changed. The cylinder pressure is gradually increased from zero value, and the pressure is bent after the peak, and then unloaded. Since the static linear analysis is performed, the load is treated as a static load. The maximum bending force of the top beam of the frame when subjected to 3 cylinders is 1000 kN, of which 400kN is allocated to the left and right cylinders, 200kN is allocated to the middle cylinder, and the direction is vertical upward; the bottom beam is subjected to the transmission of the slider and the lower die. All the bending forces down, the direction is vertical downwards.
The frame is fixed to the ground. Although the frame is fixed by anchor bolts, the anchor bolts only limit the direction of the bottom surface translation, and have no great influence on the accuracy of structural analysis. The underside of the foot limits its full constraint, as shown in Figure 3.
Figure 3——Rack load and constraints
3.3 Grid division
Meshing is a very important step in finite element analysis. The quality of the mesh is directly related to the accuracy of the finite element calculation results, and even the result is invalid. The finite element function of the SolidWords software is used to divide the mesh and the model. Divided into 30170 units, the frame finite element model is shown in Figure 4.
Figure 4——Rack meshing
Through the calculation and analysis of SolidWords software, the Y-direction displacement and stress cloud diagram of the frame are obtained, as shown in Figure 5 and Figure 6. The results show that the maximum deformation in the Y direction at full load of the frame is 2.43 mm at the top of the top beam. In actual work, the displacement of the top beam is within the elastic deformation range of the material, which has little effect on the accuracy of the machine, so the displacement value is not paid much attention to.
Figure 5——Y-direction displacement cloud map
Figure 6——Rack stress cloud
The maximum stress of the frame is 169 MPa at the rounded corner of the C-shaped throat of the side plate, which exceeds the allowable stress of the frame material Q235 steel plate by 160 MPa. In actual work, the damaged part is just here, visible early. There is a lack of design.
In response to the deficiencies of the original design, the original design was improved.
According to the frame stress cloud diagram of Fig. 6, the maximum stress of the frame appears at the bottom corner of the C-shaped throat of the side plate. As can be seen from the characteristics of the original design (Fig. 7), the C-shaped throat of the side plate of the frame. The lower fillet radius is R120, and the upper fillet is R200. According to the actual experience, changing the fillet change to the top fillet does not affect the normal use of the press brake. After the improvement, the maximum stress of the frame is 149 MPa through software analysis, and the effect is obvious. It can be seen that with a slight optimization, the maximum stress of the frame immediately drops to within the allowable stress range of the material.
Figure 7——Original design feature
In order to pursue imperfections, continue to conduct in-depth research on the original design. The original designer also considered that the C-shaped throat of the side panel of the rack is the weakest part of the frame. For the sake of safety, the designer added a stiffener to the throat of the side panel to reduce the C-shaped throat to a certain extent. The risk of cracking at the mouth. However, from the point of view of material mechanics, increasing the reinforcing ribs does not give the maximum use value of the material. Try to eliminate the reinforcing ribs on the basis of optimizing the rounded corners and then calculate and analyze, and the maximum stress of the frame is 155 MPa. Still at the lower corner of the C-shaped throat, the maximum displacement in the Y direction is 2.54 mm. Although the maximum stress after the reinforcement of the rib is removed, it is still within the allowable stress range of the material. It can be seen that although the original design of the ribs has a certain effect, but the effect is not obvious, but a lot of raw materials and assembly and welding work hours are wasted, it can be considered to cancel. However, considering that this series of models has been produced for more than 30 years, the sales volume is nearly 10,000 units, and there are many users. If the ribs are cancelled now, users will be suspicious of cutting corners. To this end, further optimization, on the basis of not changing the weight of the machine, the material of the original rib is "transplanted" to the side plate, the reinforcing rib is removed, and the width of the side plate is appropriately widened. In this way, the maximum use value of the material is fully utilized, and the strength and rigidity of the machine are significantly increased under the condition that the weight of the machine is constant, and the increase in strength and rigidity means that the overall performance of the machine is improved.
According to the optimized design data, the prototype test was carried out. It has been proved that the optimized bending machine has achieved good results. Without changing the weight of the machine, the rigidity of the machine is increased by 20%, which can save a lot of assembly and welding time, and has good economic value. It can be seen that the traditional computing design or the experience is difficult to meet the optimization requirements. The finite element software can be used to easily optimize the design and produce the best quality products with the least amount of materials.
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