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1. Reasons for loose connection of threaded fasteners
The essence of threaded fastener connection is to keep the connected parts together through axial force. When the bolt is tightened, the attenuation of the axial force is called loosening of the threaded connection.The main reasons for the loosening are as follows.
1.1 Design defects
(1) Improper selection of bolts During the bolt tightening process, the clamping force between the connecting parts increases rapidly with the increase of the pre-tightening force, and plastic deformation begins to occur when the yield point is reached. When the clamping force increases with the increase of the bolt pre-tightening force, 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 components, the bearing load, and the safety standard to select the appropriate bolt connection.
(2) Failure to consider the anti-loose design or poor anti-loose method. During use, the bolt connection may loosen or even fall off due to vibration, high and low load changes, and impact.Therefore, effective anti-loosening 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 the bolt tightening directly determines the clamping force between the two connecting parts. Insufficient pre-tightening force will inevitably lead to the loosening of the connecting bolts and the loosening of the connecting parts.The bolt pre-tightening force 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 to tighten according to feeling and experience, resulting in insufficient pre-tightening of the bolts 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, resulting in some bolts due to the wrong tightening sequence and uneven force, resulting in inconsistent tightening of the bolts. Loose.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 dimensional accuracy of threaded holes or bolt holes is particularly important when parts are connected, and the size of the thread directly affects the size of the pre-tightening force obtained by the bolt.If the size of the bolt mounting hole is too small, it will be difficult to assemble. If the size is too large, the part surface and the bearing surface of the bolt or nut will be indented and deformed, which will cause the bolt or nut to loosen.
2. Commonly used structure of threaded fasteners to prevent loosening
According to the working principle, there are three main types, the first is friction anti-loose; the second is mechanical anti-loose; the third is permanent anti-loose.