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Causes of Looseness of Threaded Fasteners and Common Structures for Preventing Looseness
The loose connection of threaded fasteners is one of the common problems. The most commonly used fasteners are loose due to vibration, high and low load changes, impact and other reasons in the use process, which leads to the reduction of equipment accuracy and safety reliability and affects tire quality and production efficiency. In order to make better use of threaded fastener connectors, the following standard parts network will share the reasons for the looseness of threaded fastener connections and the commonly used structural measures for preventing looseness.
Causes of Looseness of Threaded Fasteners and Common Structures for Preventing Looseness
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 bolts are tightened, the axial force attenuation is called thread connection loosening. The main reasons for its loosening are as follows.
1.1 Design defects
(1) Improper bolt selection In the process of bolt tightening, the clamping force between connecting parts increases rapidly with the increase of pre tightening force. When the yield point is reached, plastic deformation begins to occur. At this time, the clamping force increases little or even unchanged with the increase of bolt pre tightening force. When the pre tightening force is increased again, the clamping force gradually decreases until it breaks. Therefore, designers need to accurately analyze and calculate the tightening torque, and fully consider the weight of components, bearing load, safety standards and other factors to select appropriate bolt connection.
(2) The anti loose design or poor anti loose method is not considered, and the bolted connection is loose or even the bolt falls off due to vibration, high and low load changes, impact and other reasons during use. Therefore, effective anti loosening measures must be taken during product design to avoid loosening and falling of bolts or nuts due to severe working conditions.
1.2 Insufficient preload
The pre tightening force of bolt tightening directly determines the clamping force between two connecting parts. Insufficient pre tightening force will inevitably lead to the looseness of connecting bolts and the looseness of connecting parts. The bolt pre tightening force shall be close to or reach the yield strength in the bolt material. However, in the actual assembly process, due to the limited strength of the operator or the mismatched model of the selected tools, the output tightening torque is insufficient, resulting in the bolts can not reach the required pre tightening force.
The design and process did not put forward specific requirements for the torque of tightening bolts, and there was no corresponding torque wrench at the assembly station. Operators often judged whether to tighten the bolts based on their feelings and experience when tightening, resulting in insufficient pretightening force of some bolts at large vibration parts and looseness.
1.3 Deformation and looseness of bearing surface
When the bearing surface of the nut or bolt is under great pressure, the bearing surface of the nut or bolt and the contact surface of the connected parts will be compressed and deformed, resulting in the reduction or even loss of the pre tightening force of the threaded fasteners, which will lead to loose connections.
1.4 Improper assembly process
For the tightening of multiple bolts with regular distribution, the assembly process did not formulate a reasonable tightening process document, and the operator carried out the tightening operation completely based on his own experience, which resulted in the inconsistent tightness of some bolts due to the wrong tightening sequence and uneven stress. For example, the installation bolts commonly distributed in four squares during the assembly process are generally tightened diagonally and crosswise to ensure that the force on the bolts is balanced as much as possible, otherwise the bolts will become loose, or even the connecting parts will be deformed due to uneven force.
1.5 Processing quality defects
The size accuracy of threaded holes or bolt holes is particularly important when connecting parts. The size of thread directly affects the pre tightening force obtained by bolts. If the size of the bolt mounting hole is too small, it will be difficult to assemble, and if the size is too large, the contact between the part surface and the bolt or nut bearing surface will produce compression deformation, which will lead to the loosening of the bolt or nut.
2. Common structures for connection locking of threaded fasteners
According to the working principle, there are mainly three types: the first is friction locking; The second is mechanical locking; The third is permanent locking].

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