Causes of Loose Connection of Threaded Fasteners

Loose connection of threaded fasteners is one of the common problems. Due to vibration, high and low load changes, and impact in the process of use, fasteners may loose, which leads to the reduction of equipment accuracy and safety reliability, and affects the tire quality and production efficiency. In order to make better use of threaded fasteners and connectors, the following standard parts net shares the causes for the connection looseness of threaded fasteners and the common structural measures to prevent looseness.


The essence of the threaded fastener connection is to keep the connected parts together by axial force. When the bolt is tightened, the axial force attenuation is called threaded connection looseness. The main causes are as follows:

1.1 Design defects

(1) Improper bolt selection. During the bolt tightening process, the clamping force between the connecting parts increases rapidly with the increase of the pre-tightening force. When the yield point is reached, plastic deformation begins to occur. At this time, the clamping force increases with as the pre-tightening force of the bolt increases, the increase is small or even unchanged. When the pre-tightening force is increased again, the clamping force gradually decreases until it breaks. Therefore, the designer needs to accurately analyze and calculate the tightening torque, and fully consider factors such as the weight of the parts, the bearing load, and the safety standard, and select the appropriate bolt connection.

(2) The anti-loosening design is not considered. Due to vibration, high and low load changes and impacts, the threaded connection may lose. Therefore, effective anti-loose measures must be implemented during product design to avoid loosening of bolts or nuts due to harsh working conditions.

1.2 Insufficient preload

The pre-tightening force of bolt tightening directly determines the clamping force between the two connecting parts. The insufficient pre-tightening force will inevitably lead to the loosening of the connecting bolts and the loosening of the connecting parts. The bolt preload should be close to or reach the yield strength of the bolt material. However, in the actual assembly process, due to the limited arm strength of the operator or the mismatch of the selected tool model, the output tightening torque is insufficient, and the bolt cannot reach the required pre-tightening force.

The design and process do not put forward specific requirements for the torque of tightening the bolts, and the assembly station does not have a corresponding torque wrench. When tightening, the operator often judges whether it is tightened based on feeling and experience, resulting in insufficient bolt pre-tightening force in some large vibration parts and loosening.

1.3 The supporting surface is deformed and loosened

When the bearing surface of the nut or bolt is under a lot of pressure, the bearing surface of the nut or bolt and the contact surface of the connected parts will be collapsed and deformed, resulting in the reduction or loss of the pre-tightening force of the threaded fasteners, resulting in loose connection.

1.4 Improper assembly process

For the tightening of multiple bolts with regular distribution, the assembly process has not formulated a reasonable tightening process document, and the operator is completely relying on personal experience to perform the tightening operation. For example, the common square-shaped installation bolts in the assembly process are generally tightened diagonally to ensure that the bolts are balanced as much as possible, otherwise the bolts will loosen, and even the connecting parts will be deformed due to uneven force.

1.5 Processing quality defects

The dimension accuracy of threaded hole or bolt hole is particularly important when connecting parts. The size of thread directly affects the preload of bolt. If the size of the bolt mounting hole is too small, it is difficult to assemble. If the size is too large, the contact between the part surface and the bolt or nut supporting surface will produce indentation deformation, which will lead to the bolt or nut loosening.