Evaluation of the anti-loosening properties of fasteners

I. Test method for assessing the anti-loosening performance

For how to assess the anti-loosening properties of fasteners. Two standard test methods are commonly used at home and abroad. The first is the fastener accelerated vibration test method (GJB 715.3--1989), and the second is the fastener lateral vibration test method (ISO 16130:2015 or GB/T 10431--2008). Both are test methods for accelerating loose fastener connections, each with its own characteristics. The first method is currently mainly used in aerospace systems in China, while the second method is used in automobiles, rolling stock, construction machinery, agricultural machinery, heavy duty, Mines, coal mine machinery, and home appliance industry use, especially for the development of new types of threaded fasteners in China's railway speed provides effective data.

Since the pure lateral force (the external force of the direction of the force applied perpendicular to the axis of the fastener) is the most severe external force that causes the threaded connection to loosen, it can cause the fastener connection to loosen in a short time. Therefore, both test methods apply a pure lateral force to the fastener. Both test methods provide reproducible results for the release properties of various fastener connections.

Compared with the second method, the loading method and vibration and impact conditions of the first method are closer to the actual use state of various fasteners. The second method does not evaluate the looseness of the loose fastener connection, while the first method provides a comparable assessment of the looseness of the various fastener loose connections. In the working state, the threaded fastener connection may lose the pre-tightening force due to the wear of the bearing surface, the compression of the joint, the overload of the bolt or the external load, etc., so that the pre-tightening force may be lost. The tight connection of the state becomes a loose connection without preload (or insufficient preload). At this time, 'the fastener itself is kept locked so that it is loosened for a certain period of time, which is essential for preventing the connected member from being broken due to separation. Therefore, the first method is indispensable in assessing the anti-loose life of the fastener loose connection.

Compared with the first method, the second method is to directly apply force to the bolt (or screw), so that it can make the fastener connection loose in a shorter time, and the test efficiency is higher. Another feature of the second method is that it can accurately measure 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 force and the number of vibrations (or time). The relationship graph, and the pre-tightening variable as a measure of the looseness of the fastener connection. This is more intuitive and accurate than the first method, and it is easier to operate. In addition, the second method can more accurately assess the influence of the twist-and-pull relationship of the fastener connection on the anti-loosening performance, which provides a more effective test path for further research and design of various new anti-loose fasteners.

Second, the test results of anti-loose performance

According to the test results of the anti-loose performance, various anti-loose methods are evaluated, which have reference value for the selection of fasteners and the anti-loose method and research design of fasteners. Although the anti-loosening properties of various fasteners are affected by various factors such as structure, size, material, coating layer, manufacturing process, installation conditions, etc., the working environment is also different, but according to the above standard test methods, Under the condition that the test conditions and the factors affecting the anti-loose performance are as consistent as possible, the evaluation of the anti-loosening properties of various fasteners and their comparison results have good credibility and comparability. Many test results and assessment conclusions in this area at home and abroad are extremely similar, and the test results are reproducible. The anti-loose ability of various fasteners in the actual working environment is basically consistent with the evaluation conclusions made through the test.

Figure 19-11 and Figure 19-12 show the results of the first method, the fixture and test conditions specified in the fastener accelerated vibration test method for the anti-loosening performance test of various fasteners. The test conditions are: vibration frequency 29 ~ 30 Hz, full amplitude (11.4 ± 0.4) mm, the corresponding acceleration is about 20g. In the test. Due to the lateral direction of the fastener in the slot of the clamp

Movement (jump range up to 19 mm). Its instantaneous impact acceleration may be as high as 50g to 80g.


Third, the test results analysis of anti-loose performance

The test results of Fig. 19-11 and Fig. 19-12 show that the pre-tightening slotted nut, the anaerobic adhesive nut and the nylon ring lock nut have the longest anti-vibration life and are subjected to long-term severe impact. After the vibration, there is still no sign of looseness. In other words. Their anti-loosening properties are best in comparative tests of various fasteners.

In the past, the traditional double nut anti-loose performance was not high. the experiment shows. Change the installation method of the double nut (see Figure 19-6) to maintain a high pre-tightening force, and its anti-vibration life can be significantly improved.

The anti-loose method of the slotted nut and the split pin belongs to the locking and anti-loose of the mechanical fixing member, and has high anti-loose reliability. However, the installation method of the slotted nut and the split pin has a significant influence on the anti-loose performance. The slotted nut that is not tightened (ie, has no pre-tightening force) or is insufficiently tightened can prevent the nut from falling off the bolt for a certain period of time under severe impact and vibration. A loose connection without pre-tightening can cause early fatigue damage to the fastener or other components being connected in a harsh working environment. The test results of Figure 9.1-I show that the slotted nut without pre-tightening force is subjected to repeated impact and shearing force in the nut groove due to the repeated impact pin and the shearing force. After being subjected to more than 40,000 times of impact and vibration, the cotter pin Break, the nut is separated from the bolt, and the connection fails. Therefore, for a slotted nut that has been subjected to severe shock and vibration for a long period of time, a reasonable preload force must be applied during installation. When the nut groove is aligned with the bolt pin hole, the nut should be pulled from the tightening direction to fasten the split pin in the nut groove to prevent the split pin from moving in the groove to prevent the connection fatigue failure.

The anti-looseness of the anaerobic rubber nut belongs to the relationship of the broken thread movement, that is, the non-detachable anti-loose, and the anti-loose performance is excellent. After the long-term vibration test, there is still no sign of loosening. Since anaerobic adhesives have a high adhesion-locking ability to threaded fasteners, it is not surprising that their anti-vibration life is very long.

Nylon ring lock nut has already shown its excellent anti-loosening performance in practical applications at home and abroad. Foreign experts even call it a lock nut that never loosens. Test results show that the nut's anti-loosening performance is much higher than other lock nuts. Its anti-looseness is so reliable because the nylon ring of the nut tightly clamps the bolt to form a highly elastic lateral compression. When using a nylon ring lock nut, the aging of the nylon ring and the influence of temperature should be considered. For the nut with this requirement, the material or formulation of the nylon should be changed to improve its anti-aging and temperature performance.

The high lock nut is an all-metal lock nut that is not rounded. The high lock nut used for the test is an aluminum alloy nut, which is matched with a steel bolt. The softer aluminum alloy absorbs external shock and vibration energy better, so it is better than the steel all-metal lock nut.

Toothed lock washers, spring washers and plain nuts without locking characteristics have a vibration-resistant life of only 1% of the nylon ring lock nut. In contrast, their anti-loosening performance is very poor, generally only used in unimportant occasions.

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