Analysis of Torque Attenuation Control Technology for High-strength Galvanized Bolts

2. Experimental research

2.1 Test materials

The test was based on the mounting bolts of a model of luggage rack brackets, and the torque attenuation and its control technology were explored. There are 8 bolts for each car. The basic properties of the bolts are shown in Table 3. The shock-proof cotton is provided at the contact surface of the installation position. As shown in Figure 2, its main function is to prevent the rigid contact of the joints from accelerating under the alternating load. Loose, reduce noise inside the car and improve NVH performance.

2.2 Analysis of test ideas

By analyzing the assembly structure, the connection form is a soft connection. During the assembly process, the high-speed rotation of the electric tightening gun causes stress concentration, and the torque is instantaneously high. The stress concentrated in the entire thread system after the tightening action is stopped Released, eventually resulting in torque decay.

For the soft connection structure, the measures to reduce the torque attenuation mainly include increasing the tightening step, that is, step-by-step tightening, with an interval of about 0.5 s. After the internal stress of the anti-shock cotton is released, the slow-tight tightening to the torque target value is performed again.

The test was divided into two comparative tests, based on the torque decay failure criterion, and the target torque value applied was 8 N·m.

In the first test, the bolts were tightened directly with the ATLAS electric tightening gun without considering the contact surface as a soft connection. The test vehicle was equipped with 1# and 2# vehicles; the second test was based on the soft connection contact. The assembly torque method is to tighten the bolts step by step to reduce the tightening speed. The test vehicles are equipped with 3# and 4# vehicles.

2.3 Analysis of test results

After a long-term road test of up to 3 months, the torque is detected by the digital torque wrench. The test data is shown in Table 4.

According to the torque attenuation criterion of Section 1.4, it is judged whether the torque attenuation is reasonable. The detection torque must be above 75% of the initial target torque, that is, the attenuation is more than 6 N·m to fully ensure the reliable connection of the connected parts. The test data of the above table is processed and drawn into a line chart, as shown in Figure 3.

analyse as below:

(1) It can be seen from the data in Table 3 that the torque attenuation of the road test after direct tightening is very serious, and some fastening points may even be loose. As for the 1# car, as many as 50% of the bolts are loose, as shown in Figure 4. . Torque attenuation is caused by the addition of anti-vibration cotton between the contact surfaces to make the connection form a soft connection.

(2) For the soft connection, the step-by-step tightening method is adopted to reduce the tightening speed. It can be seen that the road test data is greater than 6 N·m, and the torque attenuation is within a reasonable range, which can ensure effective connection. Therefore, based on the torque failure criterion, changing the tightening strategy can effectively improve the torque attenuation.

3, summary

The high-strength threaded connection quality control requirements are more stringent, and effective control of torque attenuation can ensure proper axial preload and improve the reliability, tightness and anti-loose ability of the threaded connection [6]. Based on theoretical analysis and experimental research, this paper analyzes torque attenuation control as the basic method. The main results are as follows:

(1) Based on the analysis of the bolt connection force, the bolt material utilization rate is greatly exerted, and the torque attenuation mathematical model is established within a certain friction coefficient range to study the torque failure rate of the threaded fastener;

(2) Using the torque failure criterion as the judgment condition, it is verified by comparison test that the method of step-by-step tightening or reducing the speed for the soft connection can effectively control the torque attenuation, ensure the axial pre-tightening force of the bolt, reduce the bolt failure rate, and improve the production efficiency. , to guide the production and manufacture of vehicles.

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