Loose connection of threaded fasteners is one of the common problems. The most commonly used fasteners in the process of use are loosened due to vibration, high and low load changes and impact, which leads to the reduction of equipment accuracy and safety reliability and affects the quality of tires and production efficiency. In order to make better use of threaded fastener connectors, the following network of standard parts shares the reasons for loosening threaded fastener connections and common structural measures to prevent loosening.
1.Reason for connection loosening of threaded fasteners
The essence of threaded fastener connection is to hold the connected parts together through axial force. When the bolts are tightened, the axial force attenuation is called threaded connection loosening. The main reasons for its loosening are as follows.
1.1 Design deficiency
Bolt improper selection in the bolt tightening process, connecting parts between the clamping force and rapid increase with the increase of pre-tightening force, began to occur plastic deformation when the yield point is reached, the clamping force as the bolt pre-tightening force increase with increasing small constant, even when adding pre-tightening force its clamping force until fracture gradually smaller. Therefore, designers need to accurately analyze and calculate the tightening torque, and fully consider the weight of parts, bearing load, safety standards and other factors, to choose the right bolt connection.
Failure to consider the poor anti-loosening design or anti-loosening method of the bolt connection may cause the connection loosening or even bolt falling off due to vibration, high and low load changes, impact and other reasons during the operation. Therefore, effective anti-loosening measures must be implemented in product design to avoid loose bolts or nuts falling off due to bad working conditions.
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 loosening of connecting bolts and the loosening of connecting parts. The pre-tightening force of the bolt shall be close to or equal to 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 types, the output tightening torque is insufficient, and the bolts cannot meet the required pre-tightening force.
The design and process do not put forward specific requirements on the torque of tightening bolts, and there is no corresponding torque wrench at the assembly station. Operators often judge whether to tighten bolts by feeling and experience when tightening, resulting in insufficient pre-tightening force of bolts at some parts with large vibration and loosening phenomenon.
1.3The supporting surface is deformed and lossened
When the supporting surface of nuts or bolts is under a lot of pressure, the supporting surface of nuts or bolts and the contact surface of the connected parts will collapse and deform, leading to the decrease or even loss of the pre-tightening force of threaded fasteners, and thus the connection loosening phenomenon occurs.
1.4Improper assembly process
For the tightening of multiple bolts with regular distribution, the assembly process did not formulate reasonable tightening process documents, and the operators did the tightening operation entirely according to their personal experience. As a result, some bolts were loosened with inconsistent tightness due to the incorrect tightening sequence and uneven force. For example, the installation bolts with square distribution that are common in the assembly process, the diagonal cross-tightening method is generally adopted to ensure the bolt’s balanced force as far as possible, otherwise the bolt will be loosened, or even the connected parts will be deformed due to the uneven force.
1.5Machining quality defect
The dimension precision of screw hole or bolt hole is particularly important when parts are connected. The size of screw 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. If the size is too large, the contact place between the surface of the part and the supporting surface of the bolt or nut will produce compression deformation, which will lead to the loosening of the bolt or nut.
Threaded fasteners connection is commonly used to prevent loose structure
According to the working principle, there are mainly three types, the first is friction anti-loose; The second is mechanical loosening; The third is permanent containment.
Two identical nuts are tightened to the top, so that the bolts are pulled in the screw section and the nuts are compressed, so that the double nuts are generally used for longitudinal pressing of the threaded connection pair with stable load or low speed and heavy load. The anti-loosening effect is general, and the pressure is compressed after the gasket assembly with low cost. The rebound force and notch insertion keep the pressing force and friction between the threads.
Rub force and achieve anti-loosening effect, anti-loosening effect is general, mostly used for non-key processes to connect the variable tooth type anti-loosening nut. There is a bevel at the bottom of the internal thread. When the bolts and nuts are tightened, the tines of the bolts are tightly placed on the bevel of the thread, so as to produce a large locking force. The anti-loosening effect is very good.
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Link to this article：China Hardware Helps You Understand the Reasons for Loosening of Threaded Fastener Connection and Common Structures for Anti-Loosening
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