1 Design flaws
(1) Improper selection of bolts. During the bolt tightening process, the clamping force between the connecting parts increases rapidly with the increase of the pretightening force. When the yield point is reached, plastic deformation begins. At this time, the clamping force increases with the increase of the pretightening force. As the bolt pre-tightening force increases, the increase is very small or even unchanged. When the pre-tightening force is further increased, the clamping force gradually becomes smaller until it breaks. Therefore, designers need to accurately analyze and calculate the tightening torque, and fully consider factors such as the weight of the parts, load bearing capacity, safety standards and other factors to select appropriate bolt connections.
(2) Without considering the anti-loosening design or using poor anti-loosening methods, the bolted connection may loosen or even the bolts may fall off due to vibration, high and low load changes, impact, etc. during use. Therefore, effective anti-loosening measures must be implemented during product design to prevent bolts or nuts from loosening and falling off due to harsh working conditions.
2 Insufficient preload force
The pretightening force of bolt tightening directly determines the clamping force between the two connecting parts. Insufficient pretightening force will inevitably cause the connecting bolts to loosen and lead to the loosening of the connecting parts. The bolt pretightening force should be close to or reach the yield strength of the bolt material. However, during the actual assembly process, due to the operator's limited arm strength or the mismatch of the selected tool model, the output tightening torque is insufficient, and the bolt cannot achieve the required pretightening force.
The design and process do not put forward specific requirements for the torque of tightening bolts. There is no corresponding torque wrench at the assembly station. When tightening, operators often judge whether to tighten by feeling and experience, resulting in insufficient pre-tightening force of bolts in some parts with large vibrations and loosening. .
3. The supporting surface is deformed and loose.
When the bearing surface of the nut or bolt is subjected to great pressure, the bearing surface of the nut or bolt and the contact surface of the connected parts will be deformed, causing the pretightening force of the threaded fastener to be reduced or even lost, resulting in a loose connection.
4 Improper assembly process
For the tightening of multiple regularly distributed bolts, the assembly process did not formulate reasonable tightening process documents, and the operators performed the tightening work entirely based on personal experience, resulting in inconsistent tightening of the bolts due to incorrect tightening order and uneven stress on some bolts. loose. For example, during the assembly process, the installation bolts commonly distributed in a square shape are generally tightened diagonally to ensure balanced stress on the bolts as much as possible. Otherwise, the bolts will become loose, and the connected parts may even be deformed due to uneven stress. .
5 Processing quality defects
The dimensional accuracy of threaded holes or bolt holes is particularly important when connecting parts. The size of the thread directly affects the pretightening force obtained by the bolt. If the size of the bolt mounting hole is too small, assembly will be difficult. If the size is too large, the contact between the surface of the part and the bearing surface of the bolt or nut will cause depression and deformation, causing the bolt or nut to loosen.
2. Commonly used structures for preventing loosening of threaded fastener connections
According to the working principle, there are three main types: the first is friction anti-loosening; the second is mechanical anti-loosening; and the third is permanent anti-loosening].