Fastener anti-loosening method, which should we choose?


Background

As a mechanical basic part, the trend of fasteners is to develop from structural fasteners to functional fasteners. With the overall development of the domestic automobile, high-speed rail, aviation and other industries, the requirements for the anti-loose performance of fasteners are becoming more and more stringent. At present, there are many types of anti-loose fasteners, and there is no unified guiding principle, and there is no basis in the design and selection. Therefore, it is particularly important to evaluate the anti-loosening performance of fasteners. Usually, the anti-loosing performance is evaluated by the method of theoretical calculation and experimental comparison during the design and selection.
At present, there are two main methods for testing the anti-loosening performance of fasteners:
GJB715.3-89 (Accelerated Vibration Test Method”, the method of loading method and vibration, impact conditions are closer to the actual use of various fasteners, especially in the evaluation of the anti-loosening life of fasteners An indispensable method, mainly used in the aerospace industry.
GB/T10431-2008 “Fastener Transverse Vibration Test Method”, this method can accurately measure the variable of the pre-tightening force of the fastener connection during the vibration test, describe the change process of the pre-tightening force in the test, and give the pre-tightening The graph of the relationship between force and the number of vibrations (or time), and the variable of the pre-tightening force is used as the criterion to measure the looseness of the fastener connection. The test efficiency is high, and it is applicable to all industries except aerospace.
Several commonly used anti-loose fasteners are tested and compared by the transverse vibration test method, which provides a reference for the selection of anti-loose fasteners and larger-scale tests. 1.

Contrast test

Five kinds of anti-loosening fasteners were selected for comparison in the experiment. For each type of fastener, 10 groups of samples with qualified performance indexes were selected, which are as follows:
Bolt (GB/T5782) + ordinary nut (GB/T6170) + flat washer (GB/T97.1);
Bolt (GB/T5782) + variable thread lock nut (TB/T3019);
Bolt (GB/T5782) + ordinary nut (GB/T6170) + spring washer (GB/T93);
Bolt (GB/T5782) + ordinary nut (GB/T6170) + self-locking washer (Nord-lock);
Bolt (GB/T5782) + ordinary nut (GB/T6170) + safety washer (VS type).
The bolt size selected for this test is M14, and the performance level is 10.9; the nut performance level is 10, and the flat washer is 300HV.
2. Test method
The comparative test was carried out 3000 times of lateral vibration in accordance with GB/T10431-2008, and the attenuation of the test bolt pre-tightening force was recorded in real time.
The relationship between the tightening torque T, the bolt axial pre-tightening force F and the bolt nominal diameter d in the threaded connection can be expressed by the torque coefficient: T=KxFxd(1)
Since the difference in torque coefficient K between samples will affect the test results, this test did not use the same method of specifying the tightening torque, but the same method of specifying the initial axial preload. The initial preload is the axial tension of the bolt sample at 70% yield strength, namely: F=(RpO.2xAs)x70%(2)
Where:F is the initial axial preload;Rp0.2 is the specified non-proportional elongation stress of the bolt sample, Rp0.2=640MPa, see GB/T3098.1-2010;As is the nominal stress cross-sectional area of the bolt, As=157mm, see GB/T3098.1-2010.
Judgment criteria: After 3000 times or 240s lateral vibration test, the ratio of residual preload to initial preload is not less than 70% to judge the sample’s anti-loosening performance qualified.

3. Test results and analysis
(1) Bolt + ordinary nut + flat washer
The first group of tests uses ordinary nuts and flat washers. The flat washers do not have anti-vibration and anti-loose performance in principle, so they are used as the reference group for the comparison test.
From the axial force-time curve in Figure 1, it can be seen that the axial force of 90% of the bolt specimens dropped sharply within 30s after the start of the test, indicating that the static friction pair of the bolt and the nut quickly failed within 30s. The pre-tightening force has been completely lost, causing the bolt connection to loosen.
From the test results of this group, it can be concluded that ordinary nuts and plain washers basically have no anti-loosening function under dynamic load, impact, and vibration. Therefore, the use of anti-loosing fasteners is very necessary to improve the reliability of mechanical connections.

Figure 1 The first set of preload decay curve

(2) Bolt + variable thread lock nut The second group of tests uses variable-tooth locknuts. From the axial force-time curve in Figure 2, it can be seen that after 240s lateral vibration test, the ratio of the residual axial force of sample 9 to the initial axial force is less than 70%. , The residual axial force of the remaining samples can meet the test qualification criteria, and the average value of the ratio of the residual axial force to the initial axial force is above 85%. This group of tests shows that the variable tooth type locknut has a relatively good anti-loosening Performance, can keep the thread self-locking under dynamic load, shock and vibration conditions.

Figure 2 The second set of preload decay curve

The thread of the variable tooth type locknut is different from the metric thread, and there is a 30 at the bottom of the tooth. When the internal and external threads are screwed together, the crest of the external thread of the bolt will be pressed against the inclined surface of the internal thread, so that the normal force generated when the thread is in contact is 60° with the bolt axis instead of the 30° of the metric internal thread , So that the normal pressure between the threads is changed from F/sin60° to F/sin30°. When the friction factor between the threads and the axial pre-tightening force F are the same, the friction force generated by the thread with the wedge-shaped inclined surface is much greater than that of the ordinary metric thread, which greatly improves the anti-loosening torque. In addition, the thread with a wedge-shaped inclined surface eliminates the large load bearing of the first and second meshing surfaces when the metric internal and external threads are matched. The force of the entire threaded part is uneven, and there is a phenomenon of stress concentration, which effectively improves the anti-loose and anti-vibration performance, such as As shown in Figure 3.

Figure 3 SPL thread and metric thread
Variable thread type locknuts also have some shortcomings. This thread is very sensitive to the major diameter of the bolt thread, that is, the major diameter of the bolt thread is within the tolerance range, and the torque coefficient still exists after the thread is matched with the variable thread internal thread. Relatively large dispersion rate.
The large diameter of the bolt thread will be deformed due to the contact with the inclined surface, which may cause the attenuation of the axial force during repeated use, which will affect its repeated use performance to a certain extent.
(3) Bolt + ordinary nut + spring washer
The second group of tests used common standard spring washers. Figure 4 shows that the axial preload of all test samples quickly drops to 0kN within 30s after the start of the test, that is, the threaded connection has loosened and failed.

Picture test time/SFig. 4 The decay curve of the third group of pre-tightening force
The spring washer is flattened by elastic deformation to produce the elastic reaction force, which has a continuous compensation effect on the axial force of the bolt connection pair, and prevents the bolt from loosening due to fatigue caused by continuous vibration. At the same time, when there is a tendency of loosening, the tip of the oblique mouth of the spring washer abuts against the bearing surface of the bolt or nut and the surface of the connected part, which plays a role of stopping and preventing loosening.
However, the test proved that the spring washer basically cannot prevent loosening under the conditions of strong shock and vibration. The reason is that the spring steel washer cuts an opening and then undergoes elastic deformation treatment, the elastic reaction force is greatly reduced; at the same time; The size of the washer is narrow, and the equivalent diameter of the contact surface between the bolt head and the connected part becomes smaller, which significantly reduces the frictional stopping moment of this part.
Spring washers still have the problems of easy expansion when fast tightening, and the open beveled tip is easy to damage the connection surface. Therefore, it is not recommended to use spring washers alone as an anti-loosening method in key connection parts with strong vibration and high risk of loosening in this article.
(4) Bolt + ordinary nut + self-locking washer
The fourth group of tests used double-layer tooth-shaped self-locking washers (Nord-Lock). After 240s lateral vibration, the ratio of the residual axial force to the initial axial force of the 10 groups of samples was 82.51%-87.61%, as shown in Figure 5, all Satisfy the qualified standard of anti-loose performance test.

Figure 5 The fourth group of pre-tightening force attenuation curve
Double-layer tooth-shaped self-locking washers have helical teeth meshing between the layers, and there are radial serrations on the two mounting surfaces of the washer. When tightening, the radial serrations can be embedded into the bolt or nut supporting surface and the mounting surface.
When a strong vibration bolt or nut shows a tendency to loosen, the helical teeth between the washer layers will be distorted and lifted. The slope angle a of the helical teeth is greater than the metric thread rising angle, which can make the bolt stretch to stabilize the pre-tightening force. Tension bolts increase the pre-tightening force to prevent loosening, as shown in Figure 6.

Figure 6NL Washer
The use of this washer must have a large pre-tightening force to make the connecting piece and the bearing surface of the bolt or nut produce indentations and become one body. Without the indentation, the anti-loosening function of the washer cannot be exerted, that is, this type of washer is suitable for high strength Anti-loosening of fasteners.
(5) Bolt + ordinary nut + safety washer
The fifth group of tests used safety washers (VS type). After 240S strong vibration, the average value of the ratio of the residual axial force to the initial axial force of the 10 groups of samples was about 86.7%, as shown in Figure 7. The ratio of the residual axial force to the initial axial force of each group of samples is greater than 70%, that is, all the samples meet the qualified evaluation criteria of the anti-loosening performance test.
The safety washer (Type VS) is a tapered double-sided ratchet structure (see Figure 8). The ratchet is pressed into the connecting body and the bearing surface of the bolt or nut under the action of the pre-tightening force to become one body, increasing the reversal of the joint surface Friction, and the tapered structure is elastic, which can compensate for the loss of pre-tightening force after compression, and improve the anti-loosening performance from two aspects

Figure 8 Safety washer
When fasteners are under variable load, vibration and impact conditions, choosing an appropriate and reliable anti-loose type is critical to improving product quality and operational safety. The analysis concludes:
The anti-loosening performance of standard spring washers is not reliable. Because of its simple installation method and low cost, it has been widely used in all walks of life. There is a certain risk.
The variable-tooth locknut technology was introduced from abroad. After more than ten years of domestic absorption and improvement, it has gradually adapted to the needs of the domestic rail industry. It has reliable anti-loose performance and has become a relatively common anti-loose fastener product in the rail industry.
The selected self-locking washer and safety washer are patented products of imported foreign manufacturers. Their anti-loose performance is stable and reliable, but the use cost is relatively high. If corresponding standards can be established and localization can be completed, it will be beneficial to large-scale applications.

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