Causes of loose connection of threaded fasteners and common anti loosing structures
Loose connection of threaded fastener is one of the common problems. The most commonly used fasteners are loose due to vibration, high and low load changes and impact, which leads to the reduction of equipment accuracy and safety reliability, and affects the tire quality and production efficiency. In order to make better use of threaded fastener connectors, the following standard parts network will share the causes of loose connection of threaded fasteners and the commonly used structural measures to prevent loosing.
Causes of loose connection of threaded fasteners
The essence of threaded fastener connection is to keep the connected parts together by axial force. When the bolt is tightened, the axial force attenuation is called screw connection loosening. The main reasons for loosening are as follows.
- (1) 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 remains unchanged with the increase of bolt preload. When the pre tightening force is increased again, the clamping force gradually decreases until fracture. Therefore, designers need to accurately analyze and calculate the tightening torque, and fully consider the weight of parts, load, safety standards and other factors, and select the appropriate bolt connection.
- (2) Without considering the loose proof design or poor locking method, bolt connection looses or even bolts fall off due to vibration, high and low load changes and impact. Therefore, effective anti loosing measures must be taken in product design to avoid loosening and falling of bolts or nuts due to bad working conditions.
The pre tightening force of bolt directly determines the clamping force between two connecting parts, and the insufficient pre tightening force will inevitably lead to the loosening of connecting bolts and connecting parts. The pre tightening force of bolt should be close to or reach the yield strength of bolt material. However, in the actual assembly process, the output tightening torque is insufficient due to the limited arm strength of the operator or the mismatching of the selected tool model, resulting in the bolt can not achieve 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 in the assembly station. Operators often judged whether to tighten the bolts according to their feeling and experience when tightening, resulting in the loosening of bolts in some parts with large vibration due to insufficient pre tightening force.
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 collapse, resulting in the reduction or even loss of the preload of the threaded fastener, which leads to the looseness of the connection.
Improper assembly process
For the regular distribution of multiple bolts tightening, the assembly process did not develop a reasonable tightening process documents, and the operator was completely based on personal experience to tighten the operation, resulting in some bolts due to the wrong tightening sequence, uneven stress, resulting in inconsistent bolt tightness and loosening. For example, in the process of assembly, the installation bolts with square distribution are usually tightened diagonally to ensure the stress balance of bolts as much as possible, otherwise the bolts will become loose and even the connecting parts will be deformed due to uneven stress.
Processing quality defects
The dimension accuracy of threaded hole or bolt hole is particularly important when connecting parts. The size of thread directly affects the preload of bolt. If the size of the bolt mounting hole is too small, it is difficult to assemble. If the size is too large, the part surface and the bolt or nut bearing surface will be compressed and deformed, which will lead to the bolt or nut loosening.
Common anti loosing structure of threaded fastener connection
According to the working principle, there are three types, the first is friction locking; the second is mechanical locking; the third is permanent locking.
Source: China Fasteners Manufacturer – Yaang Pipe Industry (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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