1 Introduction Loose bolt failure is a common form of failure for threaded joints. Factors such as mutual embedding of material on the contact surface, material creep, insufficient pre-tightening force, and change in friction coefficient can cause the clamping force of the bolt to decrease [1, 11]; when the bolt is subjected to alternating load, vibration or high temperature, It is easier to loose [2]. The threaded connection of traditional automobile products in China is generally based on empirical design, and there is a lack of in-depth study on the anti-loosening properties of ordinary bolts and the factors affecting the anti-loose performance. The design and manufacture of bolts lacks the control of relevant parameters during assembly, and the reliability of threaded joints is poor, which affects the safety of automobiles. The principle of the bolt's anti-loosening principle and the factors affecting the anti-loose performance are theoretically studied. The effects of these factors are verified by the lateral vibration test. The application suggestions of some high-strength bolts are proposed. 2, threaded connection anti-loose principle 2.1 Threaded connection tightening torque and loose torque calculation model In order to ensure the reliable service of the bolts, sufficient clamping force should be maintained. Most of the assembly bolts use the torque method [3]. When tightening or loosening the bolts, it is necessary to overcome the frictional torque between the thread pairs and the frictional moment between the end faces of the bolts or nuts and the bearing surfaces of the connected parts [4], but the tightening torque is not equal to Release torque, which is due to the different frictional torque between the thread pairs when tightening and loosening. Assume that when tightening the bolt, the method of tightening the bolt head is used. The formula for calculating the tightening torque T is: T=T1+T2 (1) The calculation formula of the bolt loose torque T is: It has been shown in the literature that the bolt release torque is 80% of the tightening torque [5]. Take the ordinary hexagon bolt as an example, the bolt friction coefficient is 0.2, check GB/T-16823.1-1997 to obtain the outer diameter and inner diameter of the annular contact surface of different specifications, and calculate the ratio of the loose torque to the tightening torque, such as Table 1 shows. As can be seen from Table 1, the loose torque of the coarse threaded fastener is about 80% of the tightening torque, and the loose torque of the fine threaded fastener is about 85% of the tightening torque. Threaded fasteners of the same nominal diameter, under the premise of maintaining the same clamping force, the tightening torque of the fine bolts is smaller than that of the coarse bolts, but the loosening torque is larger than that of the coarse bolts, and the clamping force is larger, The larger the difference, the higher the strength of the high-strength bolt, the bolt loosening torque can be increased by replacing the fine bolt. 2.2 Analysis of factors affecting the anti-loose performance of high-strength bolts From equation (1). According to the formula (6), the pre-tightening force of the bolt, the friction coefficient, the outer diameter of the annular contact surface of the bolt and the connected member, the inner diameter, the tooth shape and the like may cause a change in the friction torque. The greater the frictional torque, the less likely the bolt is loose and the better the anti-loose performance. In actual assembly, some factors may indirectly cause changes in these parameters. 2.2.1 Head structure of the bolt There are many types of common threaded fasteners, such as hex head bolts, hex flange bolts, hex head flat washer assemblies, hexagon head flat washers and spring washer assemblies. Due to the difference in the head structure, the outer diameter of the bolt head in contact with the connected member is different. In the ordinary threaded connection, the outer diameter of the bolt head is larger, the contact surface area is larger, and the friction torque of the bolt is larger. 2.2.2 spring washer In [6], the mechanical simulation of bolts with spring washers shows that the spring washers are easy to form partial contact in the threaded connection, the support surface is small, and the open end is easy to crush, resulting in the axial force drop of the bolt and the prevention of the spring washer. Pine performance is poor. Another literature has shown that the elastic action of the spring washer [7] can compensate for the axial force of the deformation of the connecting piece and should have a certain anti-loosening effect. The anti-loose effect of the spring washer and the application range of the spring washer require further exploration. 2.2.3 Contact surface state of the bearing surface In the actual assembly, if the bolt and the surface of the connected part are not uniformly attached, the distribution of the bolt load is affected. At the same time, the small contact surface may cause local stress concentration, crushing of the surface of the material, etc., and also affect the pre-tightening force of the bolt. The surface state of the bolt and the connected member also includes the change of the friction coefficient, and the friction coefficient directly affects the friction torque of the bolt. The thickness of the surface of the bolt, the treatment of the passivation layer, the strength grade and other factors have a great influence on the friction coefficient of the bolt [8]. Therefore, the friction coefficient must be controlled to avoid Fluctuations in the coefficient of friction result in unstable anti-loosening properties. 3. Experimental study on the key factors affecting the anti-loose performance of high-strength bolts Lateral vibration is easier to loosen the bolt than axial vibration [9]. Therefore, the lateral vibration of the test bolt is used to record the change of the clamping force of the bolt with time, and the anti-loosening performance of different test bolts can be compared. The high-strength bolts of M 1 0x 1.25-8.8 are selected for testing. The surface of the test bolts and nuts are treated in the same way. The bolts and nuts are matched with 6H/6g, and each group takes 10 bolts and nuts. The amplitude of the transverse vibration tester is (±1.0) mm, the lateral displacement measurement error is within (±1)%, and the clamping force measurement error is within (±3)%. The test was carried out in accordance with GB/T 1043 l-2008. Under the condition that the bolt meets the working load requirements, the initial pre-tightening force of the bolt can be tightened in the range of (50-75)% of the yield axial force [10]. In this test, 55%, 65%, 75% of the preload force of the bolt yielding axial force was selected, that is, 14.5kN, 16.6kN, and 19.8kN, respectively, and the bolts were tested with different head structures. The results are shown in Figures 1 to 3. In order to study the influence of friction coefficient on high-strength bolts, three kinds of hexagonal flange bolts with friction coefficient were selected for the test bolts. The surface treatment was Fe/Ep·Zn8·d2D (galvanized class D passivation layer), Fe/Ep· Zn8·c2C (galvanized C-level passivation layer), Dacromet-treated bolts, the friction coefficients are 0.25, 0.2, 0.13. The lateral vibration test was carried out separately, and the test results are shown in Fig. 6. 3.1 Influence of head structure on the anti-loosening performance of high-strength bolts The clamping force of high-strength bolts with different head structures under different initial pre-tightening forces changes with time, as shown in Figures 1 to 3. As can be seen from Figure 1, when the pre-tightening force is large, the comparison of the anti-loose performance results is: hexagonal flange bolts > hexagonal head and flat pad assembly > hexagonal head, flat pad and spring pad assembly > hexagonal head and spring pad combination Parts> Ordinary hex head bolts. As can be seen from Figure 2, when the pre-tightening force is 16.6kN, the comparison of the anti-loose performance results is: hexagonal head and spring pad assembly> hex flange bolt> hex head, flat pad and spring pad assembly> hex head peace Pad assembly> Ordinary hex head bolts. Figure 3 is consistent with the anti-loosening performance of Figure 2, but the axial clamping force of the ordinary hex head bolt is attenuated when the pre-tightening force is small. The vibration 60s has been reduced to 30% of the initial pre-tightening force. Bolts and nuts Significant relative rotation has occurred. It can be seen from Fig. 1 to Fig. 3 that under different initial preloading forces, the high-strength bolts of different head structures have different anti-loosening performance, and the hexagonal flange bolts are less affected by the pre-tightening force, and the anti-loose performance is relatively stable. The head bolt has almost no anti-loosening effect, and the anti-loose performance of the spring washer is unstable. As can be seen from Fig. 1, the clamping force of the hex head + spring pad assembly has a tendency to continuously decrease, and it can be seen from Fig. 2 and Fig. 3. The hex head + spring pad assembly has less damping force. When the initial preload force is high, the spring washer has no obvious anti-loose performance, but when the initial pre-tightening force is low, as shown in Fig. 2 and Fig. 3. Has a certain anti-loose effect, as shown in Figure 4. This is due to the fact that the spring washer has two states of "depression" and "non-depression" in use. It can be seen from Fig. 4 that when the spring washer is not crushed, if the threaded connection is slightly deformed, the axial force reduction of the bolt is smaller than that of the bolt without the spring washer, and has a certain axial force compensation effect. When the spring washer is in a "depressed" state under a high preload, it cannot compensate. When the spring washer is "not pressed", although it has a certain anti-loosening effect, its open end is easy to scratch the contact surface. The anti-loosening performance is unstable. 3.2 Influence of pre-tightening force on the anti-loosening performance of high-strength bolts Take the lateral vibration test results of the hexagonal flange bolts under different preloading forces, and fit the clamping force attenuation curves of different preloading forces in Fig. 5, as shown in Fig. 5. It can be seen from Fig. 5 that the initial pre-tightening force is 19.8kN, 16.6kN, 14.4kN high-strength bolt. After 120s vibration, the axial clamping force decreases by 8.0%, 11.8%, 13.9%, respectively. The greater the initial preload, the better the bolt's anti-loosening performance. For high-strength bolts, the advantage of large yield axial force can be utilized. In actual use, the axial force is increased to enhance the anti-loose performance, and the bolt strength and material utilization rate are improved. From Figure 1. It can be seen from Fig. 3 that under different initial preloading forces, the locking performance of different bolts is different. After the above theoretical analysis of the spring washer's anti-loose principle, the pre-tightening force affects the anti-loose performance of the spring washer. Affects the anti-loosening performance of ordinary hex head bolts. When the initial preload is 55% of the yielding axial force, the ordinary hexagonal head has poor anti-loosening performance, the axial clamping force is reduced by 83.6%, and the initial pre-tightening force is 75% of the yielding axial force. At the time, the axial clamping force is reduced by 13.8%. Therefore, in order to maintain the reliable connection of the hex head bolt, the range of the initial preload force should be controlled in actual use. High-strength hex head bolts are common in micro-cars, and the torque is generally set to 50 to 70% of the yield torque. It can be seen from the lateral vibration test that in the vibration environment, for the ordinary hex head bolt, the design torque is the yield torque of 55. % may cause loose bolts to fail. High torque should be designed in use, which is about 70% of the yield torque. It is not recommended to use in critical vibration environments such as engines and chassis. 3.3 Influence of surface treatment on anti-loosening performance The axial clamping force variation curve of the bolts processed by different surfaces is shown in Fig. 6. It was found that the smaller the coefficient of friction, the worse the anti-loosening performance of the bolt. It can be seen from Fig. 6 that the high-strength bolt of the galvanized Class D passivation layer has a axial force drop of 0.05% after 120 s. The clamping force has stabilized, and the Dacromet bolt with a friction coefficient of 0.13 has a 1.25% reduction in axial clamping force after 120s, and there is still a tendency to continue to decay. Through the analysis of the above test results, the friction coefficient can have a great influence on the anti-loosening performance of the bolt, which is consistent with the bolt loosening principle. The larger the friction coefficient, the larger the friction torque that the bolt can provide. For bolts of the same type, for the anti-loose performance requirements of this place, it is necessary to select a suitable surface treatment method, and it is necessary to control the dispersion of the friction coefficient, and the fluctuation of the friction coefficient will affect the stability of the bolt's anti-loose performance. 4 Conclusion (1) The tightening torque and the loose torque of the bolt are analyzed. It is found that the loose torque of the coarse bolt is about 80% of the tightening torque, and the loose torque of the fine bolt is about 85% of the tightening torque. For the bolts of the same nominal diameter, the same clamping force is maintained. The tightening torque required for the fine bolts is lower than that of the coarse bolts, but the loosening torque is higher than that of the coarse teeth, and the loosening resistance of the fine bolts is coarser. The teeth are good. (2) By summarizing the factors affecting the anti-loosening performance of high-strength bolts, it is found that the thickness of the teeth, the pre-tightening force, the head structure of the bolt, the surface state of the bearing surface, the friction coefficient, etc., can affect the friction torque provided by the bolt. It is the main factor affecting the anti-loosening performance of high-strength bolts. (3) The lateral vibration test of bolts with different head structures was carried out, and it was found that the hexagonal flange bolts have good anti-loose performance and stable anti-loose performance. The spring washer is greatly affected by the initial preload. When the initial preload is large, the spring washer does not have the anti-loosening performance. When the initial preload is small, it is lower than 65% of the yield axial force. Anti-loose performance. Ordinary hex head bolts are basically free of anti-loose | raw energy, it is not recommended to use the joints where the impact is more frequent in the car, such as the chassis, engine, etc.; and to ensure its reliable connection, the pre-tightening force should not be less than 70 % yielding axial force. (4) The pre-tightening force is an important factor affecting the bolt's anti-loose performance. The greater the pre-tightening force, the better the bolt's anti-loose performance. (5) Perform lateral vibration tests on hexagonal flange bolts of different surface treatments. It is found that the greater the friction coefficient, the better the bolt's anti-loose performance. Fluctuations in the coefficient of friction affect the bolt's anti-loosening properties. In actual use, different surface treatment methods can be selected for different anti-loose requirements. (6) High-strength bolts are widely used in the automotive industry. Through the analysis of the influence of the anti-loose performance of high-strength bolts, the actual use can be guided. For example, the bolts can be improved by increasing the pre-tightening force or friction coefficient. Performance to meet current lock-up requirements and reduce the cost of using locknuts. The use of some fasteners needs to pay attention to the conditions of use: the use of spring washers needs to control the preload force, and when the vibration is more severe, the spring washers do not have the ideal anti-loosening effect; the pre-tightening force of ordinary hexagon bolts is better. When low, the clamping force is attenuated severely. references [1] BhattacharyaA, SenA, DasS. Analysis of the anti-loose performance of bolts under vibration environment [J]. Mechanical Design and Theory, 2010, 45(8): 1215-1225. (Bhattacharya A. Sen A, Das S. An investigation on the anti-loosening characteristics of threaded fasteners under vibratory conditions [J]. Mechanism and Machine Theory, 2010, 45(8): 1215-1225.) [2] Liu Chaoying bolt connection loose analysis [J]. Modern Machinery, 2002 (1): 70. (Liu Chao—ying. Analysis of loosening of threaded fasteners [J]. Modern Machinery, 2002 (1): 70.) [3] Yan Lianggui, Ji Minggang. Mechanical design [M]. 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