The tire bolt is a very important fastener on the rear axle. Its function is to fix the hub and the wheel. The wheel assembly is used to fix the wheel assembly on the rear axle, so that the wheel can carry and transmit power. During the running of the car, if the tire bolts are loose or even broken, the driving safety of the vehicle will be seriously jeopardized. After the pickup test of a pickup truck, the problem that the rear axle tire bolt breaks and the tire falls off occurs. In this paper, through the analysis of the tightening torque of tire bolts, the causes of the bolt breakage of the rear axle tires are explored, and optimization measures are proposed to provide a reference for solving similar problems.
1 Fault car torque analysis
In this study, the wheel of a pickup truck type is an aluminum wheel, and 6 bolts are riveted on the hub. The tire bolts are not in contact with the rim, and the pressing force of the rim is provided by the matching tapered surface of the nut and the rim, and the overall distribution of the tire bolts And the load is shown in Figure 1. When the tire is in the reverse braking, the maximum braking torque of the single tire of the rear wheel is M q = 1200 N.m, and the radius of the bolt axis and the wheel axis is r 1 = 69.85 mm. In the test field feedback, after the test vehicle (2#) passed the slab road, the right rear tire bolt was completely broken, and the right rear wheel was detached. After the failure occurred, it was found that the same batch of another test vehicle (1#) also had tires. Bolt breakage problem. After receiving the fault feedback, immediately proceed to analyze the faulty wheels and tire bolts of the two test vehicles and compare the nut torque test and the results of the other wheel nuts. The comparison results are shown in Table 1:
After observing the tire bolt section and the rim bolt mounting hole have been severely deformed, combined with the nut fitting surface wear, the broken bolt with the nut has been very loose, the left rear rear wheel torque is small, and the previous reliability vehicles are not When this problem occurs, it can be preliminarily determined that the fault is that the wheel nut is loose, unable to provide sufficient pressing force, the rim and the brake drum move relative to each other, and the tire bolt is finally broken by the shear stress. It is preliminarily determined that the tire bolt mounting torque is unreasonable, which is the cause of the bolt breakage.
2 Tire bolt design check
2.1 Tire bolt and nut structure
The details of the contact between the hub and the nut are shown in Figure 2. The support surface is tapered. The tire bolt: M12×1.5×L 10.9 grade Dacromet surface treatment, tire nut: M12×1.5 bright chrome plating, fixed-twist assembly, torque requirement 120±10N.m.
2.2 Minimum clamping force calculation required for bolts
The tire bolt external load and bolt assembly position dimension shown in Figure 1 are available:
Where: M Q - tire bolt external load
μ c ——The friction coefficient of the contact surface between the brake drum and the rim, the value is 0.25
F 1 - tightening axial force of 1 point bolt
r 1 ——The torque radius of the 1-point bolt axis relative to the rim axis
K s - considering the reliability coefficient of the contact surface instability, etc., generally takes 1.2 to 1.5
In this connection pair, when the value μ c = 0.25, K s = 1.5, it is assumed that F 1 = F 2 = F 3 = F 4 = F 5 = F 6
r 1 =r 2 =r 3 = r 4 = r 5 =r 6 =69.85mm, taking the value into Equation 1, you can get:
2.3 Minimum preload force provided by the bolt
Tire bolt specifications: M12 × 1.5 × 46, grade 10.9. The structure of the support surface is shown in Figure 2.
Thread diameter: d 2 = 11.026mm, yield strength: σ 0.2 = 940MPa, friction coefficient: thread part μ G = 0.15-0.30, (hexavalent yellow zinc, without friction control lubricant), support surface μ k =0.10 —0.16, (brightness chrome). Stress cross-sectional area: A 0 = 88.1mm 2
The equivalent torque radius of the support surface is available from:
Considering the attenuation of the axial force after bolt installation, take the attenuation coefficient r=0.7, then FT min =22.1×0.7=15.47 KN according to the above calculation:
The safety factor is less than 1, and the theoretical check is unqualified. From the torque measurement result of the faulty vehicle, the torque of the left rear wheel of the tire bolt is small, the tightening torque is 120±10N.m, the required pre-tightening force cannot be provided, and the wheel nut is loose. Provide sufficient pressing force. Therefore, it is necessary to recalculate the reasonable tightening torque of the bolt and re-check with the corrected tightening torque to determine the problem that the current tire bolt solves the bolt breakage after adjusting the tightening torque.
2.4 Bolt design check after adjusting the installation torque
Dynamic yield torque of bolt: When the thread friction coefficient μ G =0.15, μ k =0.10, the yield axial force is:
Adjusted bolt tightening torque: To ensure that the bolt is in the elastic range after tightening, take the safety factor V=0.8, then M A =193.25×0.8=155 Nm, and correct it to 145±10 Nm according to the accuracy of the installation tool. Corrected minimum preload: when μ G = 0.3, μ k =0.16, tightening torque M Amin = 135N.m
The calibration is qualified and the safety factor is greater than 1.
According to the current bolt check situation, the following conclusions are drawn:
(1) When the tightening torque is 120±10N.m, the bolt torque check is unqualified;
(2) By adjusting the tightening torque, the safety factor can reach 1.10, and the check is qualified, but the friction coefficient of the bolt thread is 0.15-0.30. The dispersion is too large. The existing bolt manufacturer can not control the friction coefficient within the range of 0.15-0.30, and the friction coefficient is detected. The table is shown in Table 1: The test results are 0.088, 0.094, 0.093, and 0.090, respectively, and are not in the range of 0.15-0.30. The pre-tightening force required for bolt tightening cannot be provided. By communicating with the standard parts manufacturer, changing the tightening torque and changing the friction coefficient of the bolt thread can further improve the safety factor and ensure the pre-tightening force required for the bolt to be tightened.
By communicating with the standard parts manufacturer, changing the tightening torque and changing the friction coefficient of the bolt thread can further improve the safety factor and ensure the pre-tightening force required for the bolt to be tightened.
2.5 Calculation of bolt tightening torque after modifying friction coefficient
Bolt dynamic yield torque:
Tire bolt specifications: M12 × 1.5 × 46, grade 10.9. The structure of the support surface is shown in Figure 2.
Thread diameter: d 2 = 11.026mm, yield strength: σ 0.2 = 940MPa, friction coefficient: thread part μ G = 0.12-0.18, (environmental Dacromet, top coat with friction control lubricant), support surface μ k = 0.11—0.17, (trivalent color zinc + with lubricating sealer).
Stress cross-sectional area: A 0 = 88.1mm 2
The equivalent torque radius of the support surface is available from:
When the thread friction coefficient μ G =0.12 and μ k =0.11, the yield axial force is:
Bolt tightening torque: To ensure that the bolt is in the elastic range after tightening, take the safety factor V=0.8, then M A =204×0.8=163 Nm, and correct it to 155±10 Nm according to the accuracy of the installation tool.
Minimum preload force after modifying the friction coefficient: when μ G =0.18, μ k =0.16, tightening torque M Amin =145N.m
The safety factor and the adjustment torque are further improved, which is the best of the above three calibration results. See Table 2 for comparison of bolt tightening torque and friction coefficient before and after modification.
After calculation and verification, the current surface friction coefficient of the bolt is high, the dispersion is large, the minimum safety factor is 0.90, and the minimum safety factor can be increased to 1.10 when the tightening torque is 145±10 Nm. After modifying the friction coefficient of the thread, the friction coefficient is reduced and the dispersion is reduced. By optimizing the matching installation torque, the safety factor can be as small as 1.29.
However, there is a certain gap between the theoretical check and the actual situation. The theoretical check requires that the friction coefficient of the support surface calculated above requires uniform contact of the contact surface. If the contact is eccentric due to the positioning or dimensional fit problem, the friction coefficient will change. The force attenuation coefficient is 0.7, but the tightening method and the matching size may cause the axial force attenuation coefficient to be greater than this value. In order to ensure the above safety factor after installation, dimensional matching and tightening process are also required.
Therefore, according to the theoretical check results and the actual experience of the standard parts supplier, it can be seen that the bolt tightening torque is insufficient, the friction coefficient is too high, and the dispersion is too large. Unable to provide sufficient pressing force. It can be determined that the tire bolt tightening torque is insufficient, and the friction coefficient is too high, which is the main cause of the breakage of the rear axle tire bolt.
3 Improvement measures
1) Replace the tire bolt manufacturer, the surface treatment is changed to environmentally friendly Dacromet, and the friction coefficient is modified to 0.12-0.18;
2) The tightening torque is changed from 120±10 Nm to 130±10 Nm.
4 optimization verification
1) Improve the surface treatment process After modifying the friction coefficient, the factory samples are sampled, and the surface hardness, mechanical properties and thread friction coefficient of the bolts all meet the design requirements. As shown in Figure 3.
2) After the tightening torque is changed from 120±10 Nm to 130±10 Nm, the torque measurement of the wheel bolts of the new off-line vehicle is carried out. After the road test, the design requirements (120~140N·m) are met, and the torque is increased. The nut is loose. The improved torque measurement is shown in Figure 4.
After the rectification, four vehicles were inspected in the test site, and the mileage was over 12,000 km. No loose nuts or broken bolts occurred during the test. In the market problem report, the PPM value of the pickup bolt problem of the pickup truck was reduced from 357 before the improvement to 0, and the rectification effect was obvious.
5 Conclusion
After checking the tire bolts, the main reasons for determining bolt breakage are:
(1) The friction coefficient of bolt thread plating is uncontrollable, which can not meet the design requirements, and there is insufficient pre-tightening force. Through the environmental Dacromet surface treatment process, the friction coefficient is controlled within the range of 0.12-0.18, so that the thread friction coefficient can be controlled. Increase the minimum safety factor of the bolt;
(2) The tightening torque of the tire bolt is unreasonable, the pre-tightening force of the bolt is insufficient, and the sufficient pressing force is not provided, which causes the wheel nut to loosen. The bolt is broken by the shearing force, and the tightening torque is modified to 130±10 Nm. The bolt breakage problem is solved by the above two modifications.
Through the handling of this bolt breakage problem, the following two points should be focused on the development of the later tire bolts:
(1) Nut tightening torque check calculation and torque confirmation.
(2) Bolt surface treatment method and friction coefficient requirements.
references
[1] Liu Weixin. Automotive Design [M]. Beijing: Tsinghua University Press, 2001.
[2] GB/T 3098.4 Mechanical properties of fasteners.
[3] Cheng Daxian. Mechanical Design Manual (Fifth Edition): Single Line - Connection and Fastening. 2010-1-1.
Fittings For Toilet And Flush Tank
Fittings for two piece toilet:
1. inlet valve ( lower pressure inlet valve and high pressure inlet valve)
2. flush valve and accessories for one piece toilet, two piece toilet and wall hung flush tank
Direct factory with professional R&D department, product range covers Smart Toilet, Bidet cover, toilet seat, tank fittings, flush tank, flush valves, and Intelligent Toilet.
Fittings For Toilet And Flush Tank,Tank Fittings,Toilet Fittings,Toilet Flush
XIAMEN JIEENTE IMPORT AND EXPORT CO.,LTD , https://www.jieente.cn