In modern machinery, the number of fasteners accounts for approximately 60% of the total number of machine parts. Among them, threaded connections are one of the most commonly used forms of mechanical connections. Generally, threaded fastening has self-locking properties, but they may loosen under variable loads, impacts, vibrations, or when the working temperature changes significantly, resulting in a decrease in preload.
The friction coefficient of threaded fasteners is an important design parameter, which directly affects the material strength efficiency utilization and anti-loosening performance of the bolts.
Lugs with a higher friction coefficient generally have good anti-loosening performance, but the preload force obtained under the same torque is smaller. Moreover, during the tightening process, the lug will be subjected to greater shear force, and the equivalent tensile stress of the lug is more likely to exceed the strength limit of the material.
At this point, the designer often needs to increase the specification of the lug to meet the design requirements of the preload force, resulting in material waste. The bolts with a lower friction coefficient can achieve a greater preload under the same torque, but their own anti-loosening performance is poorer.
Thus, it can be seen that the selection of bolt friction coefficient is a problem of balancing the anti-loosening performance and the utilization of strength efficiency.
01 Theoretical Analysis
The loosening of bolts is mainly caused by two reasons: relaxation and loosening. Among them, relaxation refers to the phenomenon that the clamping force of the fastener will decline within 5 to 10 minutes after the assembly is completed, and it is mainly caused by two reasons: the mutual embedding of rough surfaces and the creep of the material.
Loosening refers to the phenomenon that after the assembly is completed, the fastener structure is subjected to certain time of alternating loads, and a significant relative rotation occurs between the nut and the bolt, resulting in a decrease in the preload force until the preload force disappears.
The friction coefficient, as an important design parameter for threaded fasteners, directly affects the anti-loosening performance of the threaded fasteners. According to the previous analysis of the force on the threads, due to the effect of the thread helix angle, the torque required to tighten the nut (T1) and the torque required to loosen the nut (T2) are different. Generally, the loosening torque is about 80% of the tightening torque. Therefore, the greater the loosening torque, the lower the possibility of the bolt loosening.
When the type of the thread is determined (the helix angle of the thread is determined), and the working conditions are determined (Q is determined), the loosening torque is positively correlated with the friction coefficient. Here, it is necessary to examine the absolute value of T2, that is, within a certain range, the larger the friction coefficient, the greater the loosening torque, and the corresponding anti-loosening performance of the bolt is relatively better.
02 Experimental Analysis
From the above theoretical analysis, it can be seen that the friction coefficient is an important factor affecting the anti-loosening performance of the bolt. In order to further explore the influence law of the bolt’s friction coefficient on its anti-loosening performance, experiments were designed based on the theoretical research as follows.
2.1 Experimental Materials
The subject of this study is the analysis of the influence of friction coefficient on the anti-loosening performance of high-strength bolts. Therefore, the selected experimental sample model is M10*1.25*65-8.8. The head shape of all samples is a hexagonal head flange surface.
The only difference is that the surface treatment of the bolts in each group is different (which determines the difference in friction coefficient). The specific material information is shown in the following table:
| Serial Number | Surface Treatment Method | Friction Coefficient | Number |
| 1 | Dacromet | 0.25 | 10 |
| 2 | Dacromet | 0.20 | 10 |
| 3 | Dacromet | 0.15 | 10 |
| 4 | Dacromet | 0.12 | 10 |
2.2 Experimental Conditions
All the experimental samples used throughout the experiment were provided by the same manufacturer, ensuring consistency in terms of materials and processing techniques: The experimental environment was maintained at a constant temperature (in case of occasional weather changes, the experimental schedule would be adjusted);
The frequency selected for the experimental equipment was 12.5 Hz, and the no-load amplitude was ±0.8 mm; The termination condition of the experiment was that the vibration time reached 120 seconds or the residual axial force decreased to zero.
2.3 Experimental Method
This experiment utilized a transverse vibration testing machine, which can measure the clamping force during the tightening process of the bolt, the torque on the threaded pair, and the changes in axial force measured in stages, and can provide a real-time reflection of the attenuation of axial force.
The method adopted in this experiment is based on the principle of a single variable. The only variable throughout the entire experiment is the friction coefficient of the bolt (which is reflected by different surface treatments). The experiment was divided into 4 groups, with each group conducting 10 experiments. For each experiment, new samples and tooling fixtures had to be replaced.
Each group of experiments ensures that the initial axial force is the same (19.5 kN), and records the axial force changes for 30s, 60s, 90s, and 120s respectively. Eventually, the residual ratio of the axial force for each group of bolts is obtained. Based on the analysis and comparison of the recorded data, the anti-loosening performance of the fastener can be determined.
During the experiment, the slower the clamping force decays (the larger the residual ratio), the better the anti-loosening performance; conversely, the faster the clamping force decays (the smaller the residual ratio), the worse the anti-loosening performance.
2.4 Analysis of Experimental Results
1) The experimental data were sorted out, and the average residual axial forces at each monitoring point of the bolts with different friction coefficients during the vibration process were obtained.
As the experimental time progressed, the residual axial forces of each group of bolts decreased to varying degrees, and when the friction coefficient decreased from 0.25 to 0.12, the corresponding residual axial force ratio also decreased successively. Based on the previous analysis, the corresponding bolt anti-loosening performance also decreased successively.
2) In order to more intuitively show the relationship between the axial force attenuation of the bolts and the friction coefficient, the experimental data were integrated and processed to obtain the average values of the residual axial forces at each monitoring point as a percentage of the initial axial force, and the axial force residual values of each monitoring point under different friction coefficients were combined into four curves.
The variation trends of residual ratios of axial forces at each monitoring point under different friction coefficients. By comparing the experimental data of bolts obtained under four friction coefficients, the variation trends of axial force ratios at each monitoring point under different friction coefficients can be expressed, and the following test results can be drawn:
During the tightening process of high-strength bolts (with a strength grade of ≥ 8.8), while ensuring the tightness (the initial axial force is generally about 75% of the yield axial force), the residual axial force ratios of the above four friction coefficient bolts are all ≥ 80%. This indicates that all of the above four groups of bolts have certain anti-loosening capabilities, but there are differences in the anti-loosening performance.
After experiencing 120 seconds of lateral vibration, the axial forces of the above four types of bolts all showed a trend of attenuation. However, the degree of axial force attenuation varied among the four groups of bolts.
The bolts with a higher friction coefficient (0.25) showed that the attenuation curve almost became horizontal after 30 seconds, and the axial force attenuation was relatively gentle, indicating better anti-loosening performance; while the bolts with a lower friction coefficient (0.12) had an attenuation curve that consistently showed a downward slope throughout the 120-second experiment, and the axial force attenuation was relatively rapid, indicating relatively poorer anti-loosening performance.
03 Conclusion
Through theoretical analysis, it is concluded that the friction coefficient is a crucial factor affecting the anti-loosening performance of the bolt. When the friction coefficient is high, the anti-loosening performance of the bolt is relatively good, meaning that the connection reliability in terms of anti-loosening is higher; conversely, when the friction coefficient is low, the anti-loosening performance of the bolt is relatively poor, meaning that the connection reliability in terms of anti-loosening is lower.
Of course, the factors influencing the anti-loosening performance of the bolts are not limited to the friction coefficient. Similarly, the friction coefficient does not only affect the anti-loosening performance of the bolts; it also has a direct relationship with the torque coefficient during the bolt tightening process and the conversion of torque.
In conclusion, the selection of the friction coefficient is a problem of balancing the anti-loosening performance and the utilization of strength efficiency. This article only conducted a detailed research and analysis on the influence of the friction coefficient on the anti-loosening performance of bolts. The final selection of the specific friction coefficient for bolts needs to be considered comprehensively based on factors such as the type of bolt and the application scenario.
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