As a mechanical base, the trend of fastener is from structural fastener to functional fastener. With the development of automobile, high-speed rail, aviation and other industries, the requirements for the loose resistance of fasteners are more and more strict.
At present, there are many loose proof forms of fasteners, there is no unified guiding principle, and there is no basis for design and selection. Therefore, it is very important to evaluate the loose resistance of fasteners. Usually, the performance of the fastener is evaluated by theoretical calculation and test comparison.
At present, the test of the loose resistance of fasteners at home and abroad mainly passes two methods
Gjb715.3-89 (accelerated vibration test method), the loading mode and vibration and impact conditions are more close to the actual use state of various fasteners, especially in the evaluation of the anti loose life of fastener anti loose connection, which is mainly used in aerospace industry.
Gb/t10431-2008 transverse vibration test method for fasteners, this method can accurately measure the variables of the pre tightening force of fastener connection during vibration test, describe the change process of the preload force in the test, give the curve diagram of the relationship between the pre tightening force and the vibration times (or time), and take the variable of the reduction of the preload as the criterion to measure the connection looseness of the fastener, and the test efficiency is high, All industries except aerospace are applicable.
The test comparison of several common anti loose fasteners is carried out by transverse vibration test method, which provides reference for the selection of anti loose fasteners and larger scale tests.
1、 Comparative test
Five kinds of anti loose fasteners were selected for comparison. Each fastener was selected from 10 groups of qualified samples with each performance index, respectively:
Bolt (gb/t5782) + common nut (gb/t6170) + flat washer (gb/t97.1);
Bolt (gb/t5782) + variable tooth type jam nut (tb/t3019);
Bolt (gb/t5782) + common nut (gb/t6170) + spring washer (gb/t93);
Bolt (gb/t5782) + common nut (gb/t6170) + self-locking washer (Nord lock);
Bolt (gb/t5782) + common nut (gb/t6170) + safety washer (vs type).
The bolt specification selected in this test is M14 and the performance grade is 10.9; The performance grade of the nut is 10 and the flat pad is 300hv.
2、 Test method
The comparative test was conducted 3000 times of transverse vibration according to gb/t10431-2008, and the attenuation of the pre tightening force of the test bolt was recorded in real time.
The relationship between tightening torque T, axial preload F and nominal diameter D of bolt in screw connection can be expressed by torque coefficient: T = kxfxd (1)
Because the difference of torque coefficient K among samples will affect the test results, this test does not adopt the same tightening torque, but the same method of initial axial preload is adopted. The initial preload is the axial tension at 70% yield strength of the bolt sample, i.e. f= (rpo.2xas) X70% (2)
Where:
F is the initial axial preload;
Rp0.2 is the non proportional elongation stress specified for bolt samples, rp0.2=640mpa, see gb/t3098.1-2010;
As is the nominal stress sectional area of bolts, as=157mm, see gb/t3098.1-2010.
Evaluation criteria: after 3000 times of 240s transverse vibration test, the ratio of residual preload to initial preload is no less than 70%, and the sample's anti loose performance can be judged to be qualified.
3、 Test results and analysis
（1） Bolt + common nut + flat washer
The first group of tests adopts common nuts and flat pads, which have no vibration and anti loose performance in principle, so they are used as reference group for comparative test.
From the axial force time curve in Figure 1, 90% of the axial force of bolt specimens has decreased sharply within 30s after the test, indicating that the static friction pair of bolt and nut is fast failure within 30s, and the bolt preload has been completely lost, which causes the bolt connection to loosen.
The results of the test show that the common nut and washer have no anti loose function under the condition of dynamic load, shock and vibration. Therefore, it is necessary to use the anti loose fastener to improve the reliability of mechanical connection.
Figure 1 attenuation curve of group 1 preload
（2） Bolt + variable tooth type jam nut
The second group of tests adopts variable tooth type jam nut. From the axial force time curve in Figure 2, it can be seen that after 240s transverse vibration test, except that the ratio of residual axial force and initial axial force of sample 9 is less than 70%, the residual axial force of the remaining samples can meet the criteria of qualified test, and the average value of the ratio of residual axial force to initial axial force is more than 85%, The results show that the variable tooth type jam nut has good anti loose performance, and can keep the thread self-locking under the dynamic load, impact and vibration condition.
Figure 2 attenuation curve of group 2 preload
The thread of the variable tooth type jam nut is different from that of the metric thread, and there is a 30 at the bottom of the tooth. The wedge-shaped inclined surface of the bolt will be top of the bevel of the internal thread after the internal and external thread is screwed in, so that the normal force generated when the thread contacts with the bolt axis is 60 °， Instead of 30 in metric °， The normal pressure between threads is changed from f/sin60 ° Change to f/sin30 °。 When the friction factor between threads and axial preload force F are the same, the friction force of thread with wedge-shaped inclined surface is much greater than that of ordinary metric thread, which greatly improves the anti loose torque.
In addition, the thread with wedge bevel eliminated the large bearing capacity of the first and second mating surfaces when the internal and external threads are matched with the metric system. The whole screw thread thread is not evenly stressed, and the stress concentration phenomenon exists, which effectively improves the anti loose and anti vibration performance, as shown in Fig. 3.
Figure 3spl thread and metric thread
There are some shortcomings in the nut, which has a great sensitivity to the bolt thread diameter. That is, the bolt thread diameter is within the tolerance range, and the torque coefficient after matching with the variable tooth type internal thread is still relatively large dispersion rate.
The large diameter of bolt thread will deform due to contact with the inclined surface, which may lead to the attenuation of axial force when repeated use, and to some extent, affect its reusing performance.
（3） Bolt + common nut + spring washer
The second group of tests adopts common standard spring washers. From Figure 4, the axial preload of all test samples rapidly drops to 0kn within 30s after the test, that is, the screw connection has been loose and failed.
Picture test time / S
Figure 4 attenuation curve of group 3 preload
The elastic reaction force of spring washer is flattened by elastic deformation, which can compensate the axial force of bolt connection pair continuously, and prevent the bolt from loosening due to fatigue caused by continuous vibration. At the same time, the inclined end of the spring pad is against the support surface of the bolt or nut and the surface of the connected part, which plays the role of stopping and preventing looseness.
However, the test shows that spring washer can not prevent looseness under the condition of strong shock and vibration, because the spring steel gasket cuts out an opening and then it is treated by elastic deformation, and its elastic reaction force is greatly reduced; At the same time, the size of the washer is narrow, the equivalent diameter of the contact surface between the bolt head and the connected part becomes smaller, which significantly reduces the friction and withdrawal moment of this part.
Spring washer is easy to expand and the inclined tip of opening is easy to damage the connection surface when tightening rapidly. Therefore, it is not recommended to use spring washer as a loose prevention method for key joint parts with strong vibration and high risk of loosening.
（4） Bolt + common nut + self-locking washer
The fourth group of tests adopts double tooth self-locking washer (Nord lock). After 240s of transverse vibration, the ratio of residual axial force to initial axial force of 10 groups of samples is 82.51% - 87.61%, as shown in Figure 5, all of which meet the standard of qualified anti loosening performance test.
Figure 5 attenuation curve of group 4 preload
The two-layer tooth self-locking washer layer has oblique teeth to mesh with each other, and radial serrations are on the two mounting surfaces of the washers. When tightening, radial serrations can be embedded into the support surface and installation surface of the bolt or nut.
When the bolts or nuts are loose under strong vibration, the inclined teeth between the washers will be lifted by staggered motion. The slope angle a of the inclined tooth is larger than the rising angle of the metric thread, which can make the bolt lengthen to play a stable pre tightening force. It is to increase the pre tightening force to prevent looseness by stretching the bolt, as shown in Figure 6.
Figure 6nl washer
The use of the washer must have a large pre tightening force to make the connection and bolt or nut bearing surface produce indentation and become one. Without indentation, it will not play the function of anti loosening of the washer, that is, the washer is suitable for the anti loosening of high-strength fasteners.
（5） Bolt + common nut + safety washer
The fifth group of tests adopted safety washer (vs type), and the average value of the ratio of residual axial force to initial axial force of 10 groups of samples after 240s strong vibration is about 86.7%, as shown in Figure 7. The ratio of residual axial force to initial axial force of each group of samples is more than 70%, which means all of them meet the evaluation standard of the test of anti loose performance.
Figure 7 attenuation curve of group 5 preload
The safety washer (vs type) is a conical double-sided ratchet structure (see Fig. 8). Under the action of preload, ratchet is pressed into the connecting body and the support surface of bolt or nut, and becomes one body, which increases the reverse friction force of the joint surface. At the same time, the conical structure has elasticity, and can compensate for the loss of the pre tightening force after pressing, and improve the anti loose performance in two aspects.
Figure 8 safety washer
Summary
When the fastener is in the condition of load change, vibration and shock, it is important to choose the appropriate and reliable anti loose type to improve the quality of products and the safety character of operation. The analysis shows that:
The standard spring washer has unreliable anti loose performance. It is widely used in all walks of life because of its simple installation and low cost, which is a risk.
The technology of variable tooth type locknut is introduced from abroad. After more than ten years of absorption and improvement in China, it has gradually adapted to the needs of domestic track industry. The anti loose performance is reliable, and it has become a kind of common anti loose fastener product in the track industry.
The self-locking washer and safety washer are patented products of foreign manufacturers. The lock proof performance is stable and reliable, but the cost of use is relatively high. If the corresponding standards can be established and localization can be completed, it will be beneficial to large-scale application.
