Brief Analysis of the Stress Test of Changing the Outer Plate Spring

After assembling the individual pieces of the assembly stress leaf spring in the free state according to the assembly quality requirements, tighten the spring bolt of the leaf spring with the specified tightening torque and adjust the strain gauge to the 0 position. The measured assembly stress value can be found in (unit: N/mm2): the stress in the free state, the assembly stress, the stress test in the state of the force state, in the state of the leaf spring clamping, the leaf spring assembly is pre-pressed twice, and then The initial value of the strain gauge at each point of the strain gauge is uniformly adjusted to 0 position. The load interval P=200N is the applied load, increasing from P=0 to P=7800N and decreasing from P=7800N to P=0.

The strain is measured for each strain point on the first piece, the second piece and the sub-spring, and the measured strain values ​​of the loading and unloading are converted into corresponding stress values ​​of the respective measuring points. The results of the analysis can be concluded from the variation of the strain along the length of the sheet. After the stress value appears at the center hole of the leaf spring, the secondary stress is high in the direction of the sheet length near 200 mm from the center hole. This trend is basically consistent with theoretical calculations.

There is a certain gap between the theoretical calculated stress and the measured stress. The reason for the analysis is: (1) The ideal isometric beam, the variable contour line should be a unilateral parabolic type. However, the variable contour of the tested leaf spring is a single-sided trapezoidal straight line type. (2) Comparing the parabolic type and the tested spring trapezoidal linear line structure, the thickness of the corresponding distance from the center hole in the variable section section, the latter being smaller than the former; and the end size (thickness) of the detected spring It is also much smaller than the theoretical parabolic cross-section spring.

When the applied load P=800N and P=1600N: there is a difference between the contact force point and the original design contact force point. The reason is that due to the padding between the reeds, the contact length between the reeds is less than the design contact length value of 650 mm. The effect of pre-stressing: pre-stressing in the free state and pre-stressing in the clamping state along the length of the reed The change of direction: in the free state, the change of the pre-stress of each reed in the longitudinal direction basically maintains a linear relationship; in the clamped state, the change of the pre-stress of each reed in the longitudinal direction is nonlinear. This situation is caused by the fact that the force condition after the clamped spring is clamped becomes more complicated.

Discussion on the defects of the leaf spring design Because the second piece and the first piece are under the same working condition, the two pieces are subjected to a large stress than one piece. Theoretical calculations show that the stress at the root is 1.5 times different. After the assembly stress and the measured stress are synthesized, 45mm and 150mm from the center hole, when the full load P=3600N and the verification load P=7800N, the stress difference between the first piece and the second piece is 1.886-2.455 times. From this, it can be judged that the service life of the second piece is low.

Since the measured end section of the spring is smaller than the thickness of the ideal isostatic beam end section, the actual stiffness of the second trapezoidal section end section is smaller than the end section of the isostress beam under the same load. Therefore, in this section, the stress of the first sheet will be increased, so that the first sheet is easily broken in this section.

Analysis of dangerous fracture section: The second piece is the most stressful at 45 mm from the center hole, and the cross section changes most during the reed rolling, so this part is the most dangerous fracture section. The second piece has a second highest stress value at 187 mm from the center hole, which is the second dangerous fracture section. After the first piece of maximum stress point is combined with the assembly clamping stress, the maximum stress point appears at 150250 mm from the center hole, which is the first piece of dangerous fracture.

These deficiencies in the design will leave the leaf springs with insufficient normal service life. By re-adjusting the structural parameters and design parameters, the original design defects can be compensated to some extent and the service life of the leaf spring can be improved. To this end, we propose the following improvements: 1. Adjust the stress distribution of each piece. Try to reduce the stress difference between the first piece and the second piece under the same load.

2. Adjust the thickness of the first or second piece. Make the thickness of one or two pieces as close as possible. When the thickness of the second piece is thinned and the stiffness cannot meet the requirements, consider adding a third piece of the main spring.

3. When you are unwilling to adjust the thickness of one or two sheets, you can increase the secondary reed arc height and appropriately lower the second arc height.

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