What is the reason for the loose fastener connection?

The essence of the fastener connection is to maintain the connected parts together by radial force. When the bolt is tightened, the loss of radial force is called the loosening of the screw connection. The main reasons for the loosening are as follows: 1. Defects in the design scheme (1) Unreasonable use of bolts. In the bolt tightening process, the clamping force between the connected components expands rapidly with the increase of the pre-tightening force. When the yield limit is reached, the shape changes. At this time, the clamping force changes little or even does not change with the increase of the bolt pre-tightening force. When the pre-tightening force is added, the clamping force slowly shrinks until it breaks. Therefore, the designer must carry out accurate analysis and calculation of the tightening torque, and comprehensively consider various factors such as component weight, load bearing, and testing standards, and choose the bolt connection that suits him. (2) Failure to take into account the design of the fasteners or the poor fastening method. During use, the bolted connection may loosen or even fall off due to vibration, changes in bearing capacity and impacts. Therefore, efficient fastening measures should be implemented during product appearance design to avoid bolts or nuts from falling off due to extreme working conditions. 2. Insufficient pre-tightening force The pre-tightening force of bolt tightening directly determines the clamping force between two connected parts. Insufficient pre-tightening force will inevitably lead to loosening of the connecting bolts and the loosening of the connecting parts. The bolt preload should be close to or reach the tensile strength of the bolt material. However, in the actual installation process, due to the limited leg strength of the operator or the inconsistent model and specification of the selected special tools, the derived tightening torque is not enough, and the bolts cannot reach the standard pre-tightening force. The design plan and process did not put forward specific requirements for the torque of the bolts. There is no corresponding torque wrench at the grinding station. When tightening, the operator often judges whether it is tightened by intuition and experience, resulting in insufficient pre-tightening force of the bolts in some places with large vibrations and falling off. 3. Deformation and loosening of the support surface When the support surface of the nut or bolt is under great pressure, the contact area between the support surface of the nut or bolt and the connected parts will appear indentation deformation, resulting in reduced or even missing pre-tightening force of threaded fasteners, and even loose connection. 4. The machining process is unreasonable. Maijin has not formulated a reasonable tightening process specification for the tightening of several regular bolts, and the machining process has not formulated a reasonable tightening process procedure. The operator carries out the tightening work completely based on practical experience, resulting in some bolts being unbalanced due to the wrong tightening sequence and resulting in inconsistent bolt tightness and loosening. If the commonly used square mounting bolts are used during the installation process, the method of diagonal cross tightening is generally used to ensure that the bolt bearing force is balanced as much as possible, otherwise the bolts will be loosened, and even the connected parts will be deformed due to uneven pressure. 5. Hidden dangers in production and processing quality The machining accuracy of threaded holes or bolt holes is very important when connecting parts. The standard size of the thread is directly related to the pre-tightening force obtained by the bolt. If the size of the bolt screw hole is small, it will be difficult to install. If the size is slightly larger, the contact between the surface layer of the part and the support surface of the bolt or nut will also produce depression deformation, resulting in loosening of the bolt or nut.