The pre-tightening force of the threaded fastener connection is to make the threaded fastener connection receive a force in advance before it bears the working load, and this pre-stressed force is the pre-tightening force. Appropriate preload is an important prerequisite for enhancing the reliability and tightness of fastener connections. If the pre-tightening force does not meet the specified requirements, there will be gaps or relative slippage between the connected parts after being loaded, resulting in loosening of the fasteners, and even the whole machine cannot work normally. If the pre-tightening force is too large, it will cause artificial damage to the parts. For example, if the O-ring seal is used, if the pre-tightening force is too large, the O-ring will be squeezed and the seal will fail.
Improper preload on threaded fastener connections can have the following consequences:
1. Static failure of connecting parts of threaded fasteners
If the threaded fasteners are over-tightened, that is, the pre-tightening force is too large, it will cause artificial damage to the parts, the bolts may be broken, the connecting parts may be crushed, seized, twisted or broken, and the thread teeth may also be damaged. The shape is cut and tripped.
2. The connected parts slip, separate or loosen the fasteners
For the common bolted connection under lateral load, the pre-tightening force generates positive pressure between the connected parts, and relies on friction to resist the external load, so the size of the pre-tightening force determines its bearing capacity. If the pre-tightening force is insufficient, the connected parts will slip, causing the connected parts to be dislocated, skewed, and wrinkled, and the bolts may be sheared. For the bolted connection under axial load, the pre-tightening force produces a compressive force on the joint surface, and the residual pre-tightening force after the external load is the compressive force when the joint surface is working. If the pretightening force is insufficient, it will lead to the loosening of the joint surface, and even lead to the serious consequences of the separation of the two connected parts. At the same time, the insufficient pretightening force will also cause strong lateral vibration, resulting in the loosening of the nut and other phenomena.
3. Fatigue failure of bolts
Improper preload will in most cases cause the bolt to fail due to fatigue. Although reducing the preload force can reduce the average value of the total load of the cyclic change on the bolt, it increases the load amplitude, so the overall effect is mostly to reduce the fatigue life of the bolt and cause fatigue damage.
Therefore, the range of preload force must be determined in the assembly process. China Standard Parts Network prompts to determine the scientific and reasonable range of pre-tightening torque according to the importance of the connected parts, the stress situation, 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 abide by the torque requirements specified by the process during assembly. Only in this way can the reliability of the threaded connection and the fatigue resistance of the coupling be truly improved.
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