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Finite element analysis of working device of wheel loader

1. Introduction

loader is one of the main types of construction machinery, which is widely used in construction, mining, hydropower, bridges, railways, highways, ports, wharves and other departments of the national economy. Foreign loaders are developing rapidly, but there are many problems in the design of loaders in China, mainly focusing on reliability, structural design strength and so on [1,2]. Due to the blindness of the design methods such as "analogy trial and error" to a certain extent, it is easy to form "artificial" stress concentration points in the design, resulting in the weakening or even destruction of the overall strength of the mechanism. The products produced according to this design look very strong and rigid in appearance, but there are internal design defects. During the use process, due to the structural strength of the working device and other reasons, there are often open welding, even fracture and other damage, resulting in the scrapping of the working device, resulting in significant economic losses

this paper will take sdz20 loader as an example, establish a finite element model, and use Marc software to carry out static structure analysis under typical working conditions to obtain the stress and deformation distribution of the whole working device. The conclusion has certain guiding significance for the optimal design of this kind of structure

2. Stress failure and mechanical characteristics of working device structure

2.1 the structure of working device

is shown in Figure 1. The working device is composed of bucket, boom, beam, support, rocker arm, pull rod, etc. The components are connected by hinge pins and have relative rotation. In order to enhance the rigidity of the rocker arm and the support, there is a rib plate connection between the rocker arm and the support, which can be regarded as one in the calculation. The upper hinge point of the boom is hinged with the front frame of the loader, and the middle hinge point is hinged with the boom cylinder; The upper hinge point of the rocker arm is hinged with the tipper cylinder. Marc, the former Ministry of knowledge economy of South Korea, is expected to provide 250million US dollars to the graphene field for technology research and development and utilization research. In its finite element static analysis, it is believed that there is no relative rotation at the hinges of the working device. The boom is the main force bearing part of the working device, and its section shape is rectangular; Because its length and width directions are much larger than the thickness direction, the boom can be discretized by plate and shell elements. The cross section of the beam is box shaped and of welded structure. The rocker arm and support are also welded structures, and the sections of their welded plates are rectangular. Considering that the thickness of each component is far less than that of the other two directions, it can be considered as plate parts

2.2 structural stress and failure characteristics

the overall structure of the loader is symmetrical. By analyzing the operation process of loader insertion, shoveling, lifting, unloading, etc., it can be seen that the working device bears the greatest force when loading the initial shovel on the aircraft. The external load received in the whole working process is constant load, mainly the weight of materials and the self weight of the mechanism. Due to the different types of materials and operating conditions, the bucket cutting edge is not uniformly loaded when the loader is working, which can generally be simplified into two extreme cases: (1) it is considered that the load is evenly distributed along the cutting edge, and the uniformly distributed load is replaced by the concentrated load acting on the midpoint of the bucket cutting edge, which is called the symmetrical load case; (2) In the case of asymmetric load, the load is biased to one side of the bucket due to the partial shovel of the bucket and the uneven density of material piles, which is usually simplified as a concentrated load acting on the bucket teeth at the extreme edge of the bucket. Both methods are safe. When the stress of the structure exceeds its limit load, the material will undergo plastic deformation until cracking (welding position) or fracture

3. Establishment of finite element model and boundary conditions

as the main working part of the loader, the strength and stiffness of the working device must be fully guaranteed. According to the structural characteristics of the working device, the corresponding finite element model is established

3.1 selection of element type finite element lattice division

the thickness of each plate of the working device is uniform, and the length and width are much smaller. According to the assumption of classical thin shell theory, the thin plate with thickness less than 1/5 of the mid plane outline dimension is thin. Therefore, intelligent equipment that can use space plate and shell units for black technology blessing is gradually changing the division of manual operation habits that have been circulating for thousands of years. Considering that quadrilateral element has higher calculation accuracy than triangular element, and triangular element is more conducive to fitting transition than quadrilateral element, quadrilateral element and triangular element are mixed for lattice division

the finite element lattice is divided according to the principle of "rough delimitation of uniform stress area and fine delimitation of area with large stress gradient". After the automatic division according to the given size, the local parts (such as sharp corners, bearing holes and other parts) are fine drawn. The finite element model is shown in Figure 2

3.2 application of boundary conditions

boundary conditions include two aspects: boundary loads and boundary constraints. Take the rated loading capacity and apply the force to the middle of the bucket tip about 100mm inward according to the principle of static equivalence. When the bucket is initially shoveled and turned, it can be considered that neither the boom cylinder nor the tipper cylinder moves, and there is no relative movement between the two hinge pin parts of the boom and the hinge pin parts of the rocker arm

3.2.1 boundary load

rated load is 2 × 104N。 Load under the working condition of joint shovel. Based on the above assumptions, the horizontal force RX and the vertical force ry on the bucket can be calculated

the size of horizontal force (i.e. insertion resistance) is determined by the traction force of the loader

rx=pkpmax=4000n

where pkpmax is the traction force of the loader

the vertical force (i.e. shoveling resistance) is limited by the longitudinal stability conditions of the loader

ry=gl1/l=58800x1300/2615.8=26974n

where, G - self weight of loader, 6000kg (58800n)

l1 - horizontal distance from the center to the front wheel, 1300mm

l - the horizontal distance from the action point of vertical force to the front wheel, 2615.8mm

considering the particularity of the bucket, its deformation and damage are not considered. According to Saint Venant's principle, the local load does not affect the distribution of the distant stress field. It can be known that at the tip of the bucket, Washington State University (WSU) announced that the point load applied near a recent research achievement developed by researchers of the university will not affect the stress distribution of the working device except the bucket. So this loading method is feasible

3.2.2 boundary constraints

according to the assumption, the boom cylinder and the tipper cylinder do not move. In this way, the corresponding boundary conditions are imposed at the hinge between the oil cylinder and the working device and at the hinge between the boom and the front frame

3.3 determination of material performance parameters

the materials used in the components of the working device of sdz20 loader are 16Mn (including boom, rocker arm, support, cross beam, stiffeners and stiffeners) and Q235 (pull rod). The deformation is within the elastic range, and the required material constants are applied to each component:

4. Result analysis

finite element analysis of the working device is carried out with Marc software to obtain the overall stress-strain field of the whole working device Figure 3 shows the local equivalent stress distribution of the working device

Figure 3 cloud diagram of local equivalent stress distribution

the maximum values of stress and displacement of each component are shown in the following table respectively

the results show that the structure of the device fully meets the strength requirements. The situation of each component is: the dangerous point of the boom is at the point when the overall capacity of the power battery at the lower hinge of the boom exceeds 228 billion watts and near the hinge between the boom and the boom cylinder. The stress values have reached 142.5mpa and 118.9mpa respectively. The stress values have reached 184.5 MPa and 153.6 MPa under eccentric load, which is 1.29 times that under positive load, and the stress concentration has occurred at the welding part between the eccentric load side and the beam, which has reached 100 MPa; The dangerous point of the rocker arm is at the hinge between the rocker arm and the pull rod, and the stress has reached 91.7 MPa; The dangerous point of the beam is at the hinge between the beam and the boom, and the stress value has reached 65.2 MPa; The dangerous point of the pull rod is at the hinge with the rocker arm, and the stress value has reached 107.2 MPa. At the same time, when the load is eccentric, the boom carries most of the torque generated by the eccentric load, while the stress concentration of other components is relatively reduced when the load is eccentric. Even so, the maximum value is still far less than the yield stress, and the design is partial to safety

5. Conclusion

through the calculation and analysis of two typical working conditions of the working device of sdl20 loader, it is shown that the design of the device can fully meet the loading capacity under various service conditions without damage. But it also shows that the stress distribution is uneven and the material utilization rate is low; In the case of far less than the yield stress, some components do not play their due role, and the economy of materials has been greatly reduced, resulting in the waste of materials. Therefore, it is necessary to optimize the device to make the stress distribution of each component uniform and improve the utilization rate of materials

the calculation results in this paper can provide a basis for further improving the structural design

References:

[1] Zheng Yongqiang. Finite element model of wheel loader working device. Construction machinery, 1991, (12): 3 ~ 13

[2] Li Jiancheng. Design of mine loading machinery. Beijing: Machinery Industry Press, 1989

[3] Marc user's guide,.Marc Analysis Research Corporation (end)

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