Loose connection of black metal screw fasteners is one of the common problems. Quality and production efficiency. 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. 1.2 Insufficient pre-tightening force The pre-tightening force of the bolt tightening directly determines the clamping force between the two connecting parts. 1. 1. (2) Failure to consider the anti-loose design or poor anti-loose method. When the clamping force increases with the increase of the bolt pre-tightening force, the increase is small or even unchanged. Insufficient pre-tightening force will inevitably lead to the loosening of the connecting bolts and the loosening of the connecting parts. . Commonly used anti-loosening structures for threaded fasteners are divided into three main types according to the working principle, the first is friction anti-loosing; the second is mechanical anti-loosening; the third is permanent anti-loosing]. When the supporting surface of the nut or bolt is subjected to a large amount of pressure, the supporting surface of the nut or bolt and the contact surface of the connected part will be compressed and deformed, resulting in the reduction or even loss of the pre-tightening force of the threaded fastener, thereby A loose connection has occurred. When the bolts are tightened, the attenuation of the axial force is called loosening of threaded connections.If the size of the bolt mounting hole is too small, it will be difficult to assemble. 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.5 The accuracy of the threaded hole or bolt hole size is particularly important when the parts are connected with processing quality defects.