CN100383410C - Tapered High Fatigue Strength Bolts - Google Patents

Abstract

The invention belongs to a conical high fatigue strength bolt. The technical scheme realized is: the bolt is made into an inverted truncated cone shape from its root to the end, and the taper α of the inverted truncated cone shape is 2.560-3.850°. The invention not only reduces the stress concentration, but also effectively reduces the local bending stress at the thread root of the bolt, thus significantly improving the axial stress distribution at the thread root, especially when the bolt bears eccentric load, its fatigue life is improved more obvious. The present invention is also very simple in the manufacturing process, only one milling step is added, and the processing cost will not be increased much, but the fatigue strength of the bolt is greatly improved.

Description

Tapered High Fatigue Strength Bolts

Field

The invention belongs to the field of mechanical parts, in particular to a conical high fatigue strength bolt.

Background technique

Bolt connection is a connection form widely used in machinery manufacturing and mechanical equipment installation, engineering structure, railway, and automobile. Its damage will cause economic losses to varying degrees. Especially in some important connecting parts, such as the tire bolts used to connect the wheel and the wheel hub on the front and rear axles of the car, once it breaks, it will cause unimaginable consequences.

In terms of railway transportation, whether a railway locomotive can run safely depends on the strength and reliability of the steel rails. The rails are connected to the sleepers by bolts, and the strength of the bolts directly affects the safety and reliability of the connection between the rails and the sleepers. With the continuous development of my country's railway transportation, a situation of high density, large axle load and high speed has gradually formed. The simultaneous increase of speed and axle load has increased the load on the rail, which has intensified the damage to the rail bolts. This undoubtedly puts forward higher requirements on the performance of rail bolts.

The bolt connection has the advantages of simple structure, convenient disassembly and assembly, and easy adjustment. However, the stress concentration factor at the root of the ordinary triangular thread is relatively large, and the axial stress distribution at the root of the thread is uneven, which seriously affects the connection strength of the bolt. Therefore, reducing the stress concentration at the thread root and improving the stress distribution are the main ways to improve the bolt strength.

Through various understandings and inquiries, it is known that the current high-strength bolt connections at home and abroad mainly use the following methods:

1. Increase the fillet radius at the root of the thread to reduce stress concentration;

2. Increase the small diameter of the bolt thread and reduce the axial tensile stress of the thread;

3. Change the connection structure of bolts and nuts, thereby changing the local stress state of the connection between bolts and nuts.

For the first two methods, due to the limitation of the thread profile, there is a limit to increasing the fillet radius at the root of the thread and increasing the small diameter of the bolt thread, which cannot be changed arbitrarily. For the third method, due to its high manufacturing cost, it is rarely used in the bolted connection of railway rails with a very large amount of nuts.

Contents of the invention

The object of the present invention is to overcome the deficiencies of the prior art and provide a conical high fatigue strength bolt which can effectively improve the fatigue strength of the bolt.

The purpose of the present invention is achieved like this:

The technical solution of the conical high fatigue strength bolt is: the bolt is made into an inverted truncated cone shape from its root to its end.

Moreover, the bolt root is made into an inverted truncated cone shape from the root of the bolt, and it can also be any position between the stop point of the screw thread made on the bolt and the bolt root.

Moreover, the screw thread made on the bolt is also made into a threaded cone surface at the same time, and the taper α=2.560～3.850° of the inverted truncated cone.

Advantage and positive effect of the present invention are:

Applying the finite element theory, the contact analysis of ordinary bolts and new high-fatigue strength bolts is carried out. The structure shows that: under the action of tensile load, the maximum axial tensile stress on the bolt is mainly concentrated at the thread root of the contact surface between the nut and the connected part. Nearby, the reason is that this part is the part with the largest axial tensile stress and local bending stress. However, with the application of such high fatigue strength bolts, since the thread height at the part of the maximum axial tension becomes shorter, this not only reduces the stress concentration, but also effectively reduces the local bending stress at the thread root of the bolt, thus significantly improving the thread root axis. The fatigue life of the bolt is improved more obviously when the stress distribution is oriented, especially when the bolt is subjected to eccentric load.

In addition, ordinary bolts and new high fatigue strength bolts were subjected to fatigue tests on a 2000KN electro-hydraulic servo fatigue testing machine. The test results showed that the fatigue life of the new high fatigue strength bolts was more than 2.5 times that of ordinary bolts.

The present invention is also very simple in the manufacturing process, only one milling step is added, and the processing cost will not be increased much, but the fatigue strength of the bolt is greatly improved.

Description of drawings

Fig. 1 is a half-sectional view of the structure of the present invention;

Fig. 2 is a schematic structural view showing the taper angle in Fig. 1;

Fig. 3 is a half-section schematic diagram of a position between the end point of the screw thread made on the bolt and the root of the bolt as the starting point of the inverted conical frustum.

Detailed ways

The novel high-fatigue-strength bolt of the present invention is based on the existing bolts, and the bolt 1 is made into an inverted truncated cone from the root to the end of the bolt (see Fig. 1 and Fig. 2). The taper range of the inverted truncated cone It is: taper α=2.560～3.850°, the thread made by the bolt therefore also forms a threaded cone surface 2 . In addition, any position between the stop point of the thread made on the bolt and the root of the bolt can also be used as the starting point of the inverted conical frustum (see arrow A in Figure 3). The manufacturing process of the present invention is also relatively simple, and it is to cut out a threaded cone surface according to a given taper on the basis of ordinary bolts, and then make it into an inverted truncated cone shape through milling steps.

The present invention takes a bolt with ordinary triangular thread as an example, and is equally applicable to other types of bolts.

Compared with the existing bolt manufacturing technology, the present invention is not complicated in manufacturing technology, therefore, the simple change of the bolt structure will not bring expensive manufacturing cost, on the contrary, the service life of the bolt is greatly improved.

Through the physical coefficient test of the present invention, in the fatigue damage of the bolt, 65% occurred near the thread root of the contact surface of the nut and the connected part, and the stress concentration was also obtained near the thread root of the contact surface of the nut and the connected part through finite element calculation. The coefficient is as high as 3.89. Therefore, adopting the bolt of the present invention can effectively reduce the local bending stress of the screw thread and improve the fatigue strength of the bolt.

Physical parameter detection test: the selected bolt material is 20MnTiB, the elastic modulus is 210GPa, Poisson's ratio is 0.28, the yield strength is 940Mpa, and the tensile strength is 1040MPa. The thread on the bolt adopts ordinary triangular thread, and the root of the thread has a round transition .

The bolt and the new high fatigue strength bolt were tested on the 200KN electro-hydraulic servo fatigue testing machine developed by Changchun Testing Machine Research Institute. The test results are shown in the following table:

Fatigue test results

It can be seen from the test results in the above table that the fatigue life of the novel high fatigue strength bolt is more than 2.5 times that of the ordinary bolt, which is enough to embody the advanced nature of the present invention.

Claims (3)

1. A conical high-fatigue-strength bolt, characterized in that: the bolt is made into an inverted truncated cone from its root to the end, and the taper of the inverted truncated cone is α=2.560～3.850°. 2. The tapered high-fatigue-strength bolt according to claim 1, characterized in that: the inverted conical truncated cone is made from the root of the bolt, or it can be between the stop point of the thread made on the bolt and the root of the bolt any position. 3. The tapered high-fatigue-strength bolt according to claim 1 or 2, characterized in that: the thread made on the bolt is also made into a threaded cone surface at the same time.

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