How to determine the tightening torque of threaded fasteners

In order to give full play to the effectiveness of threaded fasteners while meeting the requirements of use, the method of pre-tightening force-torque characteristic curve for tightening torque calculation is proposed. In the analysis process, through theoretical deduction and experimental analysis, the relationship between the equivalent tensile stress and the actual tensile stress in the tightening process is obtained; and the thread friction torque and the total tightening torque are obtained when the threaded fastener is tightened to yield.

Than the unchanging conclusion. The pre-tightening force-torque characteristic curve method is used to calculate the tightening torque of the load-bearing bolts of an automobile engine, and the comparison with the actual torque value shows that the method can better exert the effectiveness of the bolts.

As one of the four major connection methods, threaded connection plays a decisive role in assembly due to its simple structure, reliable connection, and convenient assembly and disassembly. Among the fastening methods of threaded connections, the torque method is still the most frequently used method due to its low cost, easy use and detectability.

Regarding the tightening torque of automotive threaded fasteners, developed countries such as Europe and the United States have formulated corresponding standards. In recent years, my country has also formulated
However, due to the uneven development of the domestic automotive industry, there is a certain gap between the automotive industry standards and the actual production conditions of the enterprise. Many companies have to formulate corporate specifications for the torque assembly process of threaded fasteners. The traditional torque setting methods can be summarized into two types: one is to apply torque manually.

Test the torque coefficient at a fixed point to obtain the average value and standard deviation of the torque coefficient of the test batch of products. And as the guidance data for the batch of products to twist. The disadvantage is that only one point of data reflects the preload-torque relationship in the entire loading process, resulting in low credibility. The second is to measure the thread friction coefficient and the friction coefficient of the supporting surface, and then obtain the torque range through theoretical calculation. The disadvantage is that the torque coefficient is calculated solely by the friction coefficient, ignoring the influence of other factors on the torque coefficient, and the tightening torque calculated by this is reliable The degree is still not high.