Through benchmarking analysis, combined with relevant standard documents and calculations, the paper analyzes the selection of heavy truck wheel nuts. It mainly analyzes the wheel nut failure form, wheel nut structure and wheel nut assembly process lamp, thus effectively improving the wheel nut. The phenomenon of high after-sale failure rate reduces the after-sales failure rate from 1% to less than 0.1%.
1. Wheel nut failure mode and failure analysis
The failure modes exhibited by the wheel nut in the aftermarket are mainly due to the occurrence of breakage and slippage of the bolts that match it, and eventually lead to major quality problems such as tire shedding. The pattern pictures are as follows:
Figure 1 Figure 2 Figure 3
From July 2014 to June 2015, the wheel bolt failures were as follows: a total of 4,298 after-sales failures, a bolt claim amount of 1.075 million yuan, and a wheel nut claim amount of approximately 90,000 yuan.
After the failure of the Luotai bolt filament, the brake drum crack, the abnormal wear of the friction plate, the rim break, and the wheel reducer failure occurred.
1.1 Why do the wheel bolts slip and break?
After the failure mode analysis from July 2014 to June 2015, the fault phenomenon of threaded teeth forming obvious axial tensile deformation accounted for about 60%, and the fracture failure mode accounted for about 40%, of which the fracture surface of the rod was obvious. The failure with fatigue cracks accounts for about 25%, and the fracture surface of the threaded part accounts for about 75% of the fatigue cracks.
The bolts are calculated according to the VDI2230 standard as follows:
Table 1
It can be seen from the above calculation results that the safety margin of the fatigue strength of the bolt portion is large, indicating that the fracture is "loose and then broken" when the fracture fails.
The main influencing factors of failure are listed as follows: the slip wire fault mainly starts from the first buckle of the thread, and the slip is caused by the process influence factors, including the friction coefficient is too large, the stick slip is generated after the fitting, or the friction coefficient is small, resulting in the axial force. Too large, the thread teeth are plastically deformed.
Considering the design factors, the specifications of the M22 bolts are too small, not suitable for domestic use conditions, or the screwing length is insufficient, resulting in excessive load at the thread, resulting in plastic deformation failure.
The analysis of the fracture fault is first loose and then broken. The main impression factors are excessive friction coefficient, insufficient axial force, unstable friction coefficient, uneven axial force, or unreasonable structure of bolt and nut joints, and poor relaxation performance. The list of icons is as follows:
Figure 4
2, three major OEM comparison
Through the same method, the three major OEMs' after-sales claims are estimated by the same method. The situation is shown in Table 2.
Table 2
As can be seen from Table 2, compared with other OEMs, Shaanxi Automobile has a high claim amount. Through further analysis of the structure of the three products, the specific structure and surface treatment process of the product are shown in Table 3.
table 3
The conclusion is that the friction coefficient of the automobile wheel nut of Shaanxi Automobile is too large, and the effective screwing length of the thread is the smallest. Further, the original German MAN wheel nut is designed. The nut is designed with a “thin wall threaded†structure, and the first few bolts are evenly distributed. .
3. Analysis of the influence of friction coefficient and surface technology
Friction coefficient influence analysis
According to the relevant experimental data of SCHATZ, the relationship between the bolt friction subordinate and the torque is the same as the following installation torque, friction coefficient u=0.08 and u=0.16:
Figure 5
The conclusion is that the axial clamping force generated by u=0.08 is 81.8% higher than u=0.16 under the same torque.
3.1 So what is the most suitable coefficient of friction?
Take the M22×1.5 strength grade 10.9 bolt as an example. The experimental results are as follows:
Image 6
It can be seen that in order to increase the maximum allowable pre-tightening force of bolts of the same specification, the friction coefficient of the thread should be appropriately reduced. The friction coefficient of the recommended nut in the German VDI standard should not be less than 0.08, so as to avoid the problem of looseness, not more than 0.2, so as to avoid stick-slip effect and low bolt utilization. The MAN183 standard is recommended to be 0.08-0.14.
3.2 Surface Process Impact Analysis
In the case of different surface processes, 10 sets of products were taken for repeated tightening experiments. The experimental record data is as follows:
Figure 7
The conclusion of 10 times of repeated tightening: galvanized + galvanized + grease surface treatment products, the experiment verified that the friction coefficient deteriorated the most, reaching 0.09;
Surface treatment product of galvanized + galvanized + stabilizer, the coefficient of friction is reduced to 0.05;
For phosphating + phosphating surface treatment products, the friction coefficient is the most stable, less than 0.02.
4. Analysis of the influence of the nut structure
4.1 Calculation of effective thread length of thread
According to the VDI12230 standard, using the Launch Zthread software, the conclusion is Meff=21.11, and the safe effective stitch length is 14 buckles.
Figure 8
Figure 9
4.2 The first few buckles of the thread structure
It can be seen that the increase from the 11 buckle to the 14 buckle, the first buckle load ratio decreased from 23.20% to 21.50%, decreased by 7.3%, the number of buckles increased from 14 to 16, the first buckle load ratio decreased by 21.50%. It was 20.88%, and the drop ratio was only 2.7%, which was not a big drop.
Figure 10
5. Comparative analysis of several improved programs
According to the previous analysis, we draw Table 4:
Table 4
The final plan is as follows:
Solution 1: The total height of the nut 27 is changed to 29, and the cost is required to refer to the MAN structure. The effective number of buckles is 15 buckles, and the first two buckles are extended to the gasket to improve the joint strength. The cost analysis shows that the cost of the bolt is constant, the height of the nut is increased by 2 mm, the weight of the blank is increased by 0.02 kg, and the cost of steel is increased by about 120 tons per year.
Option 2: The bolt is increased by 5mm, and the total height of the nut is changed from 27 to 32. The supplier is required to refer to the MAN structure, and the effective screwing can reach 17 buckles, and the front two buckles are extended to the gasket to improve the joint strength.
The cost analysis is that the total height of the bolt is increased by 5mm, the weight is increased by 0.015kg, the height of the nut is increased by 5mm, the weight of the blank is increased by 0.05kg, and the annual cost of steel is increased by 390 tons.
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