The mechanism of different anti-loose structure of high-strength bolts for automobile chassis is analyzed. By comparing and analyzing the Junker transverse vibration test, it is found that the preset turning nut does not have the vibration anti-loose performance, and the Scheider nut and the wedge-shaped anti-loose washer are obtained. The minimum anti-loosening pre-tightening force and the accuracy of the experimental data are proved by the example application, which has reference value for the thread anti-loose design.
Threaded fasteners have the characteristics of large coupling force, compact structure and easy disassembly, and are widely used in vehicle structural joints. However, once the threaded joint fails, the structure is damaged and the safety accident occurs.
Loose thread is one of the main forms of thread joint failure. Experts and scholars at home and abroad have conducted a lot of research and exploration in this field. Hou Shiyuan summarized the research status and existing problems of loose thread connection, and there are many factors affecting the looseness of threaded joints, including bolt tension, working load (form, frequency, amplitude), working temperature and various structures. Form and size, structural material, structural surface state (coefficient of friction, roughness).
One of the most influential studies was a theory developed by Junker in 1969. Junker found that lateral vibrations are more likely to cause loose threads than axial loads, and designed the Junker lateral vibration testing machine that is still widely used today (Figure 1 ).
Figure 1 Junker lateral vibration testing machine
This paper introduces several common thread anti-loosening measures in the automotive industry. Using the Junker transverse vibration testing machine, a large number of comparative tests and commissioning tests are carried out on different threaded anti-loosening structures, and the example application is carried out according to the test data to verify the correctness of the test data. .
1. Introduction to the principle of anti-looseness of different threaded anti-loose structures
For the loosening of the thread, scholars and engineers invented different thread anti-loose measures, but the effect of different thread anti-loose measures in the actual application of the project is also uneven.
There are three main types of traditional anti-loose: friction anti-loose (spring washer, preset torque lock nut, thread coating, etc.), mechanical anti-loose (frozen pin, tandem wire, etc.), and the relationship between the broken thread pair (punching, riveting, Weld nuts, etc.).
The traditional frictional anti-loose is affected by the change of friction parameters at the joint surface, which results in poor stability. The disadvantages of mechanical anti-loose and broken thread-pair motion are low installation efficiency, cumbersome operation, inconvenient disassembly and repeated use. Its application range has limitations.
For the production mode of large-scale assembly lines in the automotive industry, threaded fasteners should have the requirements of high assembly efficiency, simple operation, easy disassembly, and reliable anti-looseness.
Thread anti-loosening measures that are widely used in the automotive industry are: standard fine threaded flange nuts, preset torque lock nuts, Schmidt locknuts, wedge lock washers, etc.
1.1 Standard fine threaded flange nut
According to the geometric principle of the thread and the force analysis, the simplified calculation formula of the loose torque of the standard thread is:
Among them, FM is the bolt tension, d2 is the thread diameter, μG is the thread friction coefficient, μK is the bolt head support surface friction coefficient, DW is the fastener support surface outer diameter, DI is the maximum diameter of the joint, P For the pitch.
In the case where the bolt tensioning force FM is constant, the smaller the thread pitch and the larger the outer diameter of the fastener supporting surface, the larger the loosening torque required to be externally provided, and the better the thread loosening performance. Therefore, the coarse thread is easier to loose than the fine thread and the hex nut compared to the flange nut.
1.2 Preset torque lock nut
There are many types of preset torque lock nuts, such as flattened closing nut (Fig. 2) and nylon nut (Fig. 3). The principle is to additionally increase the friction torque at the thread, so that the nut has a locking and anti-loose function.
Figure 2 Flattened nut Figure 3 Nylon nut
1.3 Stubbs lock nut
The main structural difference between the Schlumberg nut (Fig. 4) and the standard threaded nut is that there is a wedge-shaped bevel of approximately 30° on the large diameter of the internal thread. When the thread pair is tightened, the tip of the standard external thread is tightly placed on the wedge-shaped inclined surface of the internal thread of the Schmidt. Under the same axial force, the internal thread of the stainless steel is larger than that of the standard internal thread. Radial pressure.
Therefore, the Spirax thread overcomes the main cause of loosening of the standard thread caused by lateral vibration, that is, the lateral movement between the threads; and at the same time improves the uneven force of the teeth of the standard thread (Fig. 5).
Figure 4 Schmidt thread structure
Figure 5 Comparison between the Squirrel thread and the standard thread force
1.4 wedge lock washer
The wedge-shaped lock washer (Fig. 6) is composed of a pair of washers with tooth surfaces that are engaged with each other. The two washers are identical, the outer side is a radially dense small tooth surface with a directional direction, and the inner side is a large oblique Tooth surface.
When the threaded fastener is assembled and tightened, the wedge-shaped lock washer uses its higher hardness to reverse the direction of the outer small radial tooth surface from the unloading direction, so that the threaded fastener can be firmly engaged, so the external dynamic load The resulting looseness only creates a displacement between the wedge surfaces of the two washers.
Since the inner side flank angle "α" of the mutual engagement is larger than the angle "β" of the thread, the distance in which the washer expands in the thickness direction is greater than the displacement which can be generated in the axial direction of the bolt, and the bolt is elongated to generate a larger axial force. Prevent the rotation of the bolt or nut from loosening.
Figure 6 Structure and anti-loose principle of wedge-shaped lock washer
Figure 7 Test assembly diagram
2. Lateral vibration test
According to Junker transverse vibration testing machine, the comparison test and debugging test of different threaded anti-loose structure are carried out. The assembly diagram of the vibration test is shown in Figure 7. The bolt end is fixed and the nut end is free. The following tests are for the anti-loosening performance test of the nut. The test is carried out according to the standard DIN 65151-2002 or GB/T 10431-2008.
2.1 Lateral vibration comparison test of different threaded anti-loose structures
Table 1 Test conditions
Figure 8 Test plan and bolt axial force characteristic curve
Special note: The looseness of the 1 and 2 sets in the solutions e and f respectively is due to the fact that the outer surface of the wedge-shaped lock washer is not embedded in the test gasket (simulated substrate) surface (Fig. 9), but can be embedded in the nut flange. surface.
The hardness of the 10-level nut ranges from 295 to 353 HV, and the hardness of the test gasket 45 steel is 302-350 HBW. The test gaskets are not tested for hardness one by one before the test. It can be inferred that the hardness of the test gasket exceeds 350 HBW or even greater than The surface of the wedge-shaped lock washer, so the threaded secondary coupling is loose. This also corresponds to the use conditions of the wedge-shaped lock washer, that is, the hardness of the fastener and the substrate is lower than the hardness of the wedge-shaped lock washer.
Figure 9 Loose photos in scenarios e and f
From the experimental results in Fig. 8, it can be inferred that under the same lateral vibration conditions (frequency 30 Hz, amplitude ± 0.7 mm).
1) The residual axial force of the standard fine threaded flange nut completely disappears, and it does not have anti-vibration and anti-loose properties.
2) The remaining residual axial force of the hexagonal nylon lock nut and the three-point deformed metal lock nut indicates that the two preset torque nuts do not have anti-vibration and anti-loose properties, but can play the role of “anti-offâ€. .
3) The axial force of the Shibong nut also appears to be greatly attenuated, and the residual preload is less than 38%, that is, loose. However, under the same vibration conditions, the axial force attenuation speed of the Shibong nut is slower than that of the standard threaded nut, indicating that the performance of the Shibong thread against lateral vibration is better than that of the standard thread.
4) The wedge-shaped lock washer is made of mechanical structure to prevent looseness and is not affected by lubrication. It is more reliable and safe in the case of reasonable matching.
2.2 Squirrel nut lateral vibration debugging test
It can be seen from Table 2 that under the vibration condition of frequency 12.5 Hz and amplitude ±1.6 mm, the minimum initial pre-tightening force of the Shibong nut does not loosen is about 67kN, and five sets of strengthening tests are carried out after this condition (Fig. 10a). ), at the same time compared to the standard fine threaded flange nut (Fig. 10b). It can be seen that the anti-vibration and anti-loose performance of the Shibong nut must ensure a proper pre-tightening force.
It can be seen from Fig. 8d that the Schematic nut is loose under the initial pre-tightening force of 70kN, the frequency of 30Hz and the amplitude of ±0.7mm; however, the other conditions are unchanged, the frequency is reduced to 25Hz, the vibration is 1000 times, and the nut is applied. Not loose.
Junker did not find a significant relationship between the speed of the bolt loosening and the vibration frequency in his experiments; however, Vinogradov and Huang found that loosening may occur under dynamic excitation of certain frequencies and amplitudes.
The actual vehicle structure is subjected to a load frequency less than the test frequency of 30 Hz, indicating that the M16X1.5 (10-stage) Schematic nut can resist the self-looseness of the thread caused by the lateral vibration at most frequencies under the preload of 67 kN.
Table 2 Summary of test conditions and results
Figure 10 Vibration characteristics
3. Application examples
The main components of the balance suspension after a heavy truck (such as the large bracket and the thrust rod) are connected to the frame or axle by M16×1.5 (10.9 grade) high-strength bolts and ordinary flange nuts, and the thread loosening problem often occurs. The component carries alternating loads and vibrations, and the test has a natural frequency of about 10 Hz.
In order to improve the connection reliability of the rear suspension, the minimum initial pre-tightening force FMmin of the Schlumberg nut and the wedge-shaped lock washer measured according to the test are 67kN and 45kN, respectively, and the thread lock-proof design is carried out under this condition.
Referring to the German Society of Engineers technical standard VD2230-1, for bolt assembly, for the maximum assembly prestress σred max, 90% of the specified non-proportional elongation stress RP0.2min is usually taken. The SCHATZ bolt-on analysis system measures the thread friction coefficient μG of a stainless steel or a standard nut that matches a wedge-shaped lock washer from 0.08 to 0.14. The maximum assembly preload force FM max was calculated to be 129 kN according to the following formula.
Among them, the non-proportional elongation stress RP0.2 min=940MPa, the thread stress cross-sectional area As=167mm2, the thread diameter d2=15.026mm, the stress sectional area diameter ds=14.593mm, the thread pitch P=1.5mm, the thread friction coefficient μG min = 0.08.
The tightening factor αA is defined as the ratio of the maximum and minimum installed preload, ie αA=FM max/FM min .
Table 3 Comparison of assembly process
At the same time, according to Table 3, the two schemes have good anti-loose reliability, and pass the reliability test of the 30000km bad road of the real vehicle.
From Table 3, it can be summarized and predicted that the wedge-shaped lock washer has lower precision and more adaptability to the assembly tool than the Schlumberg nut. However, wedge-shaped lock washers are not suitable for applications where the surface of the joint is not allowed to break.
4. Summary and outlook
Based on the Junker lateral vibration test, five different anti-loose measures for heavy-duty trucks with high-strength bolts M16×1.5 (10.9) were compared and tested. It is known that the pre-set torque nut of standard thread is unreliable and can only be used. To the "anti-off" effect; the minimum initial pre-tightening force of the snapper nut and the wedge-shaped lock washer against lateral vibration loosening is 67kN and 45kN, respectively.
This data has great reference value for engineering applications and can be used as the basis for thread lock design to ensure the reliability of product design. However, this data does not apply to the threaded anti-loosening connection of the softer material such as aluminum alloy or composite material.
What can be further studied at the same time
1) The standard fine threaded flange nut resists the minimum value of lateral vibration at a higher preload.
2) Research on raw materials, stamping and heat treatment of wedge-shaped lock washers.
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