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Discussion on the finite element algorithm of the stiffness of the auxiliary engine mount

1 overview

the auxiliary engine mount is used to impose additional constraints on the inertial unbalanced force generated by the impact of the gearbox on the ground. Due to the small thickness of the rubber cushion of the auxiliary mount and the poor anti cushioning ability, it mainly relies on the rigidity of the rod after the Bush is compressed and deformed for forced limit. The auxiliary mount is mainly responsible for attenuating the impact of transmission shaft vibration on the gearbox, increasing the displacement limit of the gearbox, and reducing the torque generated by the vertical inertia force of the gearbox on the rear end face of the cylinder block. Therefore, whether the stiffness design of an engine auxiliary mount is reasonable or not directly affects the stable operation of the whole transmission system

the traditional calculation of the stiffness of the auxiliary engine mount requires first measuring the force of the transmission system acting on the auxiliary engine mount bushing, and then measuring the deformation of the auxiliary mount bar through the strain gauge to calculate the stiffness. However, because the auxiliary mount is generally rigid, the deformation value is relatively small, and the force borne by the auxiliary mount changes at any time during the re movement process, and it is not easy to measure, so the error of calculating the stiffness by the test method is relatively large, and the period is long

taking an engine auxiliary mount as an example, this paper expounds the method of solving the engine auxiliary mount by using the HyperWorks software of Altair company. At the same time, by setting the flexible instrument, it is suggested to use materials at the ambient temperature below 25 ℃ to increase the accuracy of the analysis

2 establishment of finite element model

the accuracy of finite element analysis mainly depends on the finite element model used for calculation. The model must truthfully reflect the mechanical characteristics of the component structure, so the basic principle of modeling is to subdivide the lattice as much as possible under the condition that the hardware can bear it, so that the finite element model is consistent with the geometric model as much as possible. The geometric model and torsion bar finite element model of the engine auxiliary mount assembly are shown in Figures 1 and 2

if only the torsion bar of the engine auxiliary mount is analyzed, the stress and constraints are directly loaded on the rigid bar, which will inevitably lead to the stress concentration of the torsion bar and greatly affect the accuracy of the calculation. In order to eliminate these adverse factors, the author of this paper also made the finite element treatment of some rubber parts connected with the torsion bar when calculating the engine auxiliary mount. At the same time, in order to make the results more accurate, the elastic modulus E of the rubber part is set relatively small, so that it can produce relatively large deformation to simulate the characteristics of the rubber part. The model used for calculation is shown in Figure 3, and the material is shown in Table 1. Due to the stiffness calculation, only the elastic modulus E and Poisson's ratio of the material need to be known, so a small elastic modulus can be assumed to participate in the calculation

3 calculation result

the auxiliary engine mount assembly is divided into 20907 units and 9772 nodes. In order to calculate accurately, the auxiliary mounting torsion bar is simulated with hexahedral elements, with a total of 3255 time elements without experiments for a long time; The auxiliary mount mounting support and rubber parts are simulated by tetrahedral elements, with 17628 elements; The bolt is simulated by rigid element and beam element. Since only the stiffness is calculated, the actual load is not measured by test, and only 1000N force needs to be applied at the middle installation position of the auxiliary mount to calculate the deformation of the torsion bar of the auxiliary mount with a set of tolerances specially set by the precision technology. Figures 4 to 7 show the stress and deformation nephogram of the auxiliary mount assembly. Table 2 shows the deformation and stiffness calculation results of engine auxiliary mount

4 theoretical calculation result

where MGB (plus horizontal line) is the equivalent transmission mass borne by the support point. XV is the displacement control value in the vertical direction. According to our current front and rear suspension cushion size, the vertical displacement should be controlled within 5mm. Assuming that the equivalent mass of the gearbox at the support point is 85%, and the vertical acceleration in the figure 8 auxiliary mount theoretical model diagram is between 5g and 10g, it is calculated that the weight of Shaanxi gear 9 gearbox is close to 390kg, which is 331.5kg after quantification. Considering the dynamic load, it is temporarily assumed to be 350kg

the vertical displacement control value is assumed to be 5mm. Calculate according to formula 2:

when the vertical acceleration is 5g, the vertical stiffness of the cushion is 1750n/mm

when the vertical acceleration is 10g, the vertical stiffness of the cushion is 3500n/mm

take an average value of 2625n/mm of manual hydraulic universal material testing machine

at the same time, the stiffness of 10n-01030 measured in our previous tests is between, with a maximum increase of 14% compared with 2625 n/mm, and there is little difference for the rubber cushion

from the above theoretical and experimental calculation results, it can be seen that the stiffness finite element analysis result of the auxiliary mount 2994n/mm is 14% larger than the theoretical calculation stiffness value 2625n/mm, and is consistent with the test measurement value. Therefore, it can be determined that the finite element calculation result is relatively accurate. The scheme that we use flexible rubber parts to participate in the calculation to reduce stress concentration is feasible, and can replace the test scheme

5 summary

this paper conveniently solves the stiffness of the engine auxiliary mount by using the finite element method, and tries to use the method of reducing the elastic modulus to simulate non-metallic flexible materials such as rubber, which improves the accuracy of stiffness calculation, and verifies the accuracy of the simulation results by comparing with the theoretical and experimental results. Facts have proved that the results of the finite element method are very close to the test results, so it is completely feasible to use the finite element method to solve the stiffness of various complex structures. (end)

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