Effect of Friction Coefficient on Locking Performance of Tire Bolts
1 Introduction
Heavy-duty truck axle axle bolts are used as security parts for heavy-duty truck vehicles. If the bolts and nuts are loose during use, the bolts will break and the nuts will fall, causing the axle to fail. , resulting in an accident. Therefore, the research on the anti-loosening performance of tire bolts is a problem that must be paid attention to during the design and assembly process. Will loosen the bolt mechanism, lateral vibration test and real vehicle
Three aspects of the road test are used to explain the causes of loosening, and provide a basis for solving loose loosening of bolts.
2 analysis of bolt loosening principle
In view of the vehicle failure caused by bolt breakage in the market (Fig. 1), the bolt failure member (Fig. 2) is collected, and the fracture is characterized by obvious fatigue crack. The reason for the analysis is still due to looseness of the bolt and nut, and the bolt is axially preloaded. The force is reduced, and the bending alternating load is increased, which causes cracks on the surface of the bolt and expands to the core, eventually forming a fatigue crack fracture [1].
When the bolt is tightened, there is a certain frictional force on each contact surface of the connection. The friction torque generated by these forces is the tightening torque T, which is the sum of the friction torque Ts in the thread pair and the friction torque Tw of the support surface. which is:
T = Ts + Tw (1)
Where T is the total torque; Ts is the thread torque; Tw is the bearing surface friction twist; Fs is the horizontal force required to overcome the thread resistance; Fw is the resistance acting on the support surface; F is the preload; d2 is the thread Diameter; dw is the equivalent friction diameter of the support surface; β is the angle of the thread (lead angle); P is the pitch; α is the half angle of the tooth or the angle of the tooth; μw is the friction coefficient of the support surface; μs is the friction coefficient of the thread.
During the loosening process, the first and third terms in Equation 6 become the resistance torque during the loosening process, and the second is the net resistance torque of the tightening bolt. In the case of loosening, this part becomes The assisted release torque and the release torque TL are calculated as follows:
If the release torque TL is greater than 0, the bolt will not be released automatically; however, if the release torque is less than 0, the bolt will loosen itself without applying any release torque. The condition that the bolt does not produce self-loose is called the thread self-locking condition and is calculated as follows:
It can be seen from the above formula that in the case of bolt specifications and joint structure dimensions, the self-locking condition of the thread depends on the friction coefficient μs of the thread pair, the friction coefficient μw of the support surface and the pre-tightening force F. In practical engineering applications, the total control is always used. The friction coefficient μtot is used as the quality control requirement. The total friction coefficient μtot includes the screw pair friction coefficient μs and the support surface friction coefficient μw, the thread pair friction coefficient or the support surface friction coefficient increases, and the total friction coefficient also increases. It can be seen that the greater the total friction coefficient and the pre-tightening force, the higher the reliability of the thread secondary locking, but in the bolt pre-tightening process, since the pre-tightening force and the total friction coefficient follow the formula 9, the total friction coefficient is larger, The smaller the tightening force.
Where A0 is the bolt equivalent cross-sectional area; Rp0.2min is the yield strength; d0 is the diameter of the smallest section of the bolt. The increase of the friction coefficient μs of the thread will lead to a decrease in the maximum pre-tightening force FM of the bolt. In other words, the friction coefficient of the bolt is increased and the utilization of the bolt is reduced. Under the same connection structure, the design specification of the bolt will be increased. It is also unfavorable. Therefore, in the bolt anti-loose design, it is necessary to balance various factors, and the improvement of the anti-loose performance of the threaded fastener is the balance between the total friction coefficient and the pre-tightening force [2-5].
3 test analysis
In order to further explore the anti-loose performance of the tire bolt under different axial preload and total friction coefficient, and considering the torque method tightening process using fixed torque at the assembly site, the lateral vibration test and the real vehicle road test are designed. The test is as follows.
3.1 Lateral vibration test
3.1.1 Test principle
After the bolt is tightened, the eccentric wheel is used to force one of the two connected members to be reciprocally slid by the connecting member, the bolt will be loosened, the axial force change during the bolt test, the bolt axial force attenuation rate is small, and the bolt has good anti-loose effect. .
3.1.2. Test equipment
The test selected M22×1.5-10.9 tire bolts, M22×1.5-10 tire nuts for lateral vibration test, refer to GB/T 10431-2008 “Fastener lateral vibration test method†recommended SPS-A
Bunco lateral vibration testing machine, the corresponding test principle is shown in Figure 3 [6].
3.1.3. Test conditions
a. Test frequency: 12.5 Hz;
b. Load amplitude: ± 2.0 mm;
c. Number of vibrations: 3,000 times;
3.1.4. Test design
The DOE method is used to design the bolt loosening performance test plan, and two factors are selected as the design variables. See Table 1. The axial force high level selects 70% of the guaranteed load as the axial force, and the axial force decay rate as the response after the test. variable. Considering that each set of test bolts and nuts is 8 pieces, the DOE design can be designed with full factor and 8 number of lines, and the number of tests is 22×8=32 groups. Through the experimental design software Minitab design test plan and test results are shown in Table 2.
3.1.5 Test analysis
The axial force attenuation response data obtained in the test of Table 3 was analyzed by Minitab 16. Figure 4 is a main effect diagram of the test factor. It can be seen from Fig. 4 that with the increase of the axial force pre-tightening force, the smaller the attenuation rate of the axial force after the lateral vibration test of the bolt, the better the anti-loose performance; the increase of the friction coefficient, the axial direction after the lateral vibration test of the bolt The smaller the attenuation rate of the force, the better the anti-loose performance; in comparison, the increase in the friction coefficient is more significant than the increase in the axial pre-tightening force for the anti-loose effect.
3.2 Real road test
3.2.1 Test principle
In order to further explore the anti-loose performance under the condition of different friction coefficient and axial pre-tightening force of the tire bolt and nut after the actual vehicle assembly, the concrete road test is used to study. For the axial force of the bolt, the bolt ultrasonic shaft is used. The force test equipment, after the test bolt is tightened and the axial force changes after the road test is completed, the axial force attenuation rate of the bolt is small, and the bolt has a good anti-loose effect.
3.2.2 Test preparation
a. Test vehicle: 8×4 truck, see Figure 5, the vehicle is fully loaded with 31 t, of which one shaft and two shafts are steering shafts, and three shafts and four shafts are drive shafts.
b. Test bolt and nut: specification M22×1.5, considering the accuracy of the comparison test, the test bolt and nut are installed on the three-axis and four-axis, wherein the three-axis tire bolt nut is stabilized by the friction coefficient, and the torque coefficient average is K=0.15; The shaft tire bolt nut is ordinary galvanized, and the torque coefficient is K=0.25.
c. Tightening method: Torque control, installation torque 575 N·m.
d. Road test conditions: a total of 3 000 km of gravel road, slab road, climbing road and cement road under the standard load.
e. Measuring equipment: After the tire bolt is tightened and the road test is completed, the axial force is measured by I-Bolt ultrasonic axial force testing equipment. Before the tire bolt is tested, the tire bolts need to be flattened at both ends, and the ultrasonic signal is attached to one end. The film, see Figure 6, and then the axial force calibration of the tire bolt, after the calibration is completed, the axial preload force generated by the bolt can be directly measured, as shown in Fig. 7.
3.2.3 Test data and analysis
The test data is shown in Table 3. According to the test data before and after the road test, it can be seen that:
a. The bolt and nut that have been subjected to the friction coefficient stabilization treatment are in phase
Under the same installation torque, due to the small torque coefficient, the axial pre-tightening force is greater than that of ordinary galvanized bolts and nuts;
b. With 575 N·m tightening, the axial force attenuation rate of the tire bolt nut k=0.15 (initial preload force 174 kN) after the road test is completed is smaller than the axle of the tire bolt nut k=0.25 (initial preload force 105 kN) The force attenuation rate and the anti-loose effect are good.
4 Conclusion
a. From the principle analysis, lateral vibration test and real road test, the anti-loose performance of the tire bolt and nut was studied. The ultrasonic axial force test equipment was used to verify the bolt and nut by measuring the axial force of the bolt before and after the actual road test. Anti-loose performance is a scientific method;
b. Using the DOE method to design the lateral vibration test of the 2 factor 2 horizontal bolt. Through the study between the friction coefficient and the axial preload force, the anti-loose performance of the tire bolt and nut was found. The friction coefficient is the primary factor, and the friction coefficient is larger. The better the anti-loose performance of the bolt, but the pre-tightening force and utilization rate of the bolt are degraded, which is unfavorable for the structural design;
c. Through the real vehicle test, for the axle bolts of heavy-duty trucks, according to the current assembly torque of 575 N·m, the friction coefficient is used to stabilize the tire bolts and nuts (torque coefficient k is controlled at 0.15), and the axial pre-tightening Strong tightening and better anti-loose performance.
references:
[1] Tao Chunhu. Failure Analysis and Prevention of Fasteners [M]. Aviation Industry Press, 2013, (11): 97-105.
[2] Chen Weijin. Comparative analysis of bolting technology between Germany and China's rolling stock[J]. Railway Technology Supervision. 2011, 39(4): 5-6.
[3] VDI2230-2014: Systematic calculation of highly stressed bolted joints Joints with one cylindrical bolt [R].
[4] ISO16047-2005: fastening torque of fasteners - axial preâƒload test[R].
[5] Sakai Chiji, Chai Zhilong translation. Threaded fastener joint engineering [M]. Mechanical Industry Press. 2016.
[6] GB/T10431-2008 fastener lateral vibration test method [R]. AT
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