The relationship between the flatness of the nut thread diameter and the method of reducing the stress concentration at the root of the thread and improving the stress distribution. The finite element analysis of the bolt connection, the relationship between the maximum radius of the root return arc and the main parameters of the thread is shown in Fig. l is the geometric relationship between the maximum arc radius of the root of the ordinary triangular thread and the pitch, the thread height and the flattening height at the nut thread diameter. The tensile strength is 1040MPa, the yield strength is 940MPa. The bolt length is 72mm, the nominal diameter is 24, the pitch is 3, the thread diameter is 2, the nut diameter is 40, and the screwing length is 27mm.

Under the premise of not affecting the accuracy, in order to reduce the calculation amount, the influence of the radius of the root of the thread on the maximum axial tensile stress of the bolt can be analyzed. The bolt and the nut can be used as the research object alone, and the interaction between the nut and the nut can be replaced by the contact load. According to the bolt joint structure and the characteristics of the force, the finite element model is established as shown. In addition, since the bolt and the nut are two objects in contact with each other, the force of each thread in the screwing section is different, so when the finite element model is established, it cannot be regarded as a single entity, but should be treated according to the contact problem, otherwise Analytical calculations will be erroneous and ineffective. p is the pitch; H is the theoretical height of the thread; H is the flattening height at the nut thread diameter. Since the basic tooth shape of the ordinary triangular thread is an equilateral triangle, the maximum value of the maximum corner radius of the root of the thread depends on its geometry and the size of the pitch, and cannot be arbitrarily increased under the condition that the root arc of the bolt is tangent to the side. .

The unit division should be dense and reasonable, and adapt to the stress distribution law. In the thread with a large stress gradient, in order to better reflect the stress change, a dense grid is needed, and in a region with a small stress gradient, a relatively sparse grid should be divided. When the ratio of the bolt diameter to the side length of the thread root unit is greater than 10, the calculation accuracy is not affected by the size of the unit. According to the above principle, the unit type used in this paper is a quadrilateral axisymmetric plane unit, and the unit side length at the stress concentration is less than 0.021 mm.

Applying the finite element model shown, the maximum axial tensile stress at the root of the bolt near the contact surface between the nut and the rail is analyzed. The calculation result is shown by the middle fold line MP. The improved line of the maximum axial tensile stress at the root of the bolt is shown by the meander line. Even in the case of the same radius of the root of the thread, the maximum axial tensile stress of the bolt root of the improved design is reduced compared with the ordinary rail bolt, and the maximum difference is 5 MPa, which is relatively reduced by 10%. This is because of the improved design. The minimum area of ​​the rear root of the bolt is increased while at the same time alleviating the stress concentration at the root of the thread.

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