1. Reasons for loosening of threaded fastener connection
The essence of threaded fastener connection is to hold the connected parts together by axial force. When the bolt is tightened, the attenuation of axial force is called threaded connection loosening. The main reasons for its loosening are as follows.
1.1 Design defects
Bolts (1) 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, load, safety standards and other factors to select the appropriate bolt connection.
(2) The bolt connection with poor anti-loosening design or anti-loosening method is not considered in the process of use due to vibration, high and low load changes, impact and other reasons, and the connection looseness or even bolt fall off. Therefore, effective anti-loosening measures must be implemented in product design to avoid loosening of bolts or nuts due to bad working conditions.
1.2 Insufficient preload
The pretightening force of bolt tightening directly determines the clamping force between two connecting parts. Insufficient pretightening force will inevitably lead to loosening of connecting bolts and connecting parts. The bolt preload should approach or reach the yield strength of the bolt material. However, in the actual assembly process, due to the limited arm force of the operator or the mismatched tool type, the output tightening torque is insufficient, resulting in the bolt cannot reach the required pretightening 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. When tightening bolts, operators often judge whether they are tightening by feeling and experience, resulting in insufficient pretightening force of bolts in parts with large vibration and loose phenomenon.
1.3 The support surface is deformed and loose
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 depressed and deformed, leading to the reduction or even loss of the pretightening force of the threaded fastener, thus appearing the connection loosening phenomenon.
1.4 Improper assembly process
For multiple bolt tightening with regular distribution, reasonable tightening process documents are not formulated in the assembly process, and the operators completely rely on their personal experience to tighten the bolts. As a result, some bolts are not tightened in the correct sequence, and the force is not balanced, so that the bolts are inconsistent and loose. Such as the common square distribution of installation bolts in the assembly process, generally adopt the method of diagonal cross tightening, as far as possible to ensure that the bolt force balance, otherwise the bolt will loosen, or even due to uneven force deformation of connected parts.
1.5 Machining quality defects
The dimensional accuracy of threaded holes or bolt holes is particularly important when connecting parts. The size of threaded holes directly affects the pretightening force obtained by bolts. If the size of the bolt mounting hole is too small, it is difficult to assemble; if the size is too large, the contact between the surface of the part and the supporting surface of the bolt or nut will produce compression deformation, resulting in loosening of the bolt or nut.
2, thread fastener connection anti loose common structure
According to the working principle, there are three main types, the first is friction anti-loose; The second is mechanical anti-loosening; The third is permanent anti-loosening. The common thread connection anti-loosening structure, principle and application are shown in Table 1:
| The serial number | Locking type | Locking structure | Principle and characteristics of anti-loose | application |
| 1 | Double nut | Two identical nuts are tightened to the top, so that the bolt is pulled in the screwing section and the nut is compressed, so that the longitudinal compression of the thread connection pair is general and the cost is low | Generally used for smooth load or low load occasions | |
| 2 | Elastic washer | After the washer assembly is pressed, its rebound and notch embedment to maintain the compression force and friction between threads to achieve anti-loosening effect, anti-loosening effect is general | Mostly used for non-critical process connection | |
| 3 | Variable tooth type locknut | There is a wedge-shaped bevel at the bottom of the tooth of the internal thread. When the bolt nut is tightened, the tooth tip of the bolt is tightly pressed against the wedge-shaped bevel of the thread, resulting in a larger locking force and a very good anti-loosening effect | Poor versatility | |
| 4 | Friction locking | Non-metallic insert, lock nut | The thread hole is inlaid with nylon block, when the external thread is screwed in, the nylon material has good elasticity and produces locking force, and the anti-loosening effect is very good | Widely used, but the general operating temperature is -50~120℃ |
| 5 | Full metal insert, lock nut | This type nut deforms a certain part of the nut body by non-circular closing. When the bolt is matched with the non-circular thread hole of the locking nut, the deformation of the non-circular thread produces locking force to restrain the loosening of the fastener, and the anti-loosening effect is very good | It is suitable for the parts with small installation space and limited fastener weight | |
| 6 | Nordlock, Dacro anti-loose gasket | It is composed of a pair of pre-installed washers. The inner surface of the washers is big serrated, and the outer surface is small serrated. When tightening the bolts, the outer surface of the washers bite the contact surface of the bolts and the workpiece, and the anti-loosening effect is very good | It is not suitable for the occasions where the connecting surface is not allowed to be scratched | |
| 7 | Hexagon flange bolts and nuts | Increase the diameter of the woven surface of the hexagonal flange surface, better anti-loosening effect can be obtained under a certain pre-tightening force than ordinary bolts and nuts, and the anti-loosening effect is good | Suitable for high strength (grade 8.8 and above) | |
| 8 | Lock washer | There are two kinds of external teeth and internal teeth. When the nut whose tooth shape is tightened acts in reverse, its tooth tip will be embedded on its surface to achieve the purpose of locking. The anti-loosening effect is good, and the contact surface is easy to be damaged | It is not suitable for the occasions where the connecting surface is not allowed to be scratched | |
| 9 | Mechanical locking | Hexagon slotted nuts with split pins | After the fastening is in place, the split pin will lock the nut and bolt through the pin hole of the nut slot and end, and the anti-loosening effect is very good | Suitable for variable load/vibration connection |
| 10 | Series wire | With iron wire through several holes on the head of the nut, several nuts are locked at the same time, with good anti-loose performance, requirements on the structure around the installation nut, can not be reused | It is suitable for the arrangement of more group thread connection, suitable for seldom disassembled parts connection | |
| 11 | Permanent locking | Spot welding of bolts and nuts | Use riveting method to fix the nut, the best anti-loosening effect, but can not be removed | It is suitable for parts that are not disassembled basically |
| 12 | The fastening glue | Using anaerobic adhesive coating on the surface of screw thread, the adhesive can be self-cured after the nut is tightened. The method is simple, the anti-loosening effect is good, and the economy is effective | Suitable for parts that are not often disassembled |
