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What are the consequences of unsuitable pre-tightening force for threaded fastener connections?

Update:30-07-2021
Summary:

The pre-tightening force of the fastener connection is […]

The pre-tightening force of the fastener connection is the pre-tightening force in the direction of the bolt axis line generated between the bolt and the connected part under the action of the tightening torque in the process of screwing the bolt. The improper pre-tightening force of the threaded fastener connection will bring the following consequences:

1. The static damage of the connecting parts of the threaded fasteners
If the threaded fasteners are over-tightened, that is, the pre-tightening force is too large, it will cause artificial parts to be damaged. The shape is clipped and tripped.

2. The connected parts slip, separate or the fasteners loosen
For ordinary bolted connections that bear lateral loads, the pre-tightening force generates positive pressure between the connected parts, which rely on friction to resist the external load, so the size of the pre-tightening force determines its load-bearing capacity. If the pre-tightening force is insufficient, the connected parts will slip, resulting in misalignment, skew, and wrinkles of the connected parts, and the bolts may be sheared. For bolted connections subjected to axial load, the pre-tightening force produces a pressing force on the joint surface, and the remaining pre-tightening force after being subjected to an external load is the pressing force when the joint surface is working. If the pre-tightening force is insufficient, it will cause the joint surface to loosen, and even lead to the serious consequences of the separation of the two connected parts. At the same time, the insufficient pre-tightening force will cause strong lateral vibration and cause the nut to loosen.

3. Fatigue failure of bolts
Inappropriate pre-tightening force will cause bolt failure due to fatigue in most cases. Although reducing the pre-tightening force can reduce the average value of the total cyclic load on the bolt, it increases the load variation. Therefore, the overall effect is mostly to reduce the fatigue life of the bolt and cause fatigue damage.

Therefore, the pre-tightening force range must be determined in the assembly process. China Standard Parts Network prompts to determine the scientific and reasonable pre-tightening torque range based on the comprehensive consideration of the importance of the connected parts, the force condition, the movement mode, the structural characteristics, the thread specifications and grades, the material of the connected parts and the purpose of the connection, etc.; Strictly comply with the torque requirements specified by the process during assembly. Only in this way can the reliability of the threaded connection and the fatigue strength of the connection piece be truly improved.
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