Summary:
Several important concepts about bolt fastening are introduced. The selected process and strength check method of the chassis fastening bolts are demonstrated by examples. The design and calibration process can be directly applied to the actual work. Design and strength checks have strong practical value.
As the most common and important connection method in the assembly of cars, bolts and nuts have become one of the most important research topics for mechanical coupling in countries around the world including China. The components of the chassis are the most important safety parts of the car. The key items of the bolts are more secure. Once they fail, they will directly cause a safety accident.
Therefore, the strength of the bolt must be checked. Here, based on the experience of many years of automobile manufacturing companies, the author introduces the bolt selection process and uses the torque/axial force diagram to check the strength and confirm whether the selected bolts are available. In this paper, the bolt design and strength check case is derived from the real environment of the automobile manufacturing industry. It has strong practical value for the design and strength check of the automobile chassis fasteners.
1, torque / axial force diagram
The torque/axial force diagram is to make it easier to see the relationship between the torque and the relationship between the axial force and the torque coefficient. The relationship between torque, axial force and torque factor is as follows:
T=KQd (T: Torque K: Torque Coefficient Q: Axial Force d: Nominal Diameter)
When K and d are constant, T (torque) and Q (axial force) are proportional.
1.1 Torque factor
K is determined by the coefficient of friction between the bolt mating surface and the fastener, and is usually based on the material and surface state of the bolt and the fastener being used. For the chassis parts, except for special cases, 0.23 is considered as the standard torque factor when calculating the axial force and torque. There is a tolerance of ±15% between the torque factor and the standard torque factor, which needs to be reflected in the design review.
1.2 Axial force range under torque and torque factor
Normally, the torque factor is in the range of ±15%, and the tightening torque is in the range of ±10%. The axial force range of the bolt is shown in the oblique line in Figure 1.
Figure 1 Axial force range under torque and torque factor
2, the allowable axial force of the bolt
The allowable axial force of the bolt is the axial force limit that can be used if the bolt is not damaged under the bolt size and strength (material). The formula is as follows:
Qy=σ·As (Qy: allowable axial force, σ: yield strength, As: effective area of ​​bolt)
The yield strength of a material is a node where the material is plastically deformed, but there is a certain margin of error in the production of the material. Therefore, if the axial force of the bolt is directly used, the bolt may be deformed or damaged.
Moreover, when the bolt is tightened, the force applied to the bolt becomes large due to the torsional stress, and the axial force becomes large due to the external force after the tightening. Therefore, considering these factors, a certain margin is allowed for the axial force, and the allowable axial force is usually set at 70% of the yield strength. That is, the bolt allows the axial force to be set to Qy = 0.7σ·As.
3. Selection process of bolts
The load input condition (P) in the calculation of the axial force has many kinds of conditions in different systems and different parts, so it is difficult to obtain the correct value by calculation alone. According to different parts of the analysis and test to accumulate the database of load input conditions (P), and to analyze the differences in test values, it is very important in the design. Figure 2 shows the bolt selection process. The load input conditions for the sequence 1) are from the performance analysis department of the system and the automobile manufacturing enterprise.
Figure 2 Selected process of bolts
4, bolt selection check process case
1 ) P load input conditions. P: Assume that it is 7581N.
2) Calculated value of the required axial force of Q1.
μ is the friction coefficient of the fastening part, and the chassis part is usually set to 0.2.
3) Q2 initial axial force attenuation part. Q1 calculated from the load input conditions is the calculated value of the required axial force. The thickness reduction of μm is caused between the bolt contact faces that are tightened, and the material that is stressed as time passes will loosen and the axial force is attenuated. This is the initial axial force attenuation. Usually, the axial force attenuation ratio of Q2→Q1 is 10~30%, which is the concept of safety factor.
As for how to select the ratio between 10 and 30%, it is usually based on empirical values, analytical values, experimental values, and benchmarking contents to manage the attenuation ratios of different parts. Under very bad conditions, 30% will be chosen for calculation.
The following formula is the formula for calculating Q2 from Q1 when 30% axial force decay rate is selected:
4) The required initial axial force Q (final design value). The initial axial force value also needs to reflect the axial force attenuation caused by various loads (static load, dynamic load, impact load, etc.) and system characteristics after tightening. After considering these attenuation factors, the final required initial axial force is determined. .
Regarding the axial force attenuation part, it is usually analyzed and tested according to the location for database management. If there is no relevant database, the axial force attenuation ratio of Q→Q2 is usually set to 15~25%, which is also a concept of safety factor. Choose 25% for calculations under harsh conditions.
The following formula is the formula for calculating Q from the inverse of Q2 when the 25% axial force decay rate is selected.
5) Standard torque factor selection. When calculating the axial force and torque, 0.23 is regarded as the standard torque coefficient. There is a tolerance of ±15% between the torque factor and the standard torque factor.
Upper limit of torque coefficient: K × 1.15 = 0.23 × 1.15 = 0.2645
Torque factor standard value: K=0.23
Lower limit of torque coefficient: K × 0.85 = 0.23 × 0.85 = 0.155
6) Select any bolt. This paper selects the 9.8 grade M14 bolts in Table 1 [8] for an example.
7) Calculation of the minimum torque. This step is to calculate the minimum torque required to satisfy the required initial axial force Q = 36100N obtained in the 4) sequence, according to the formula T = KQd (K = 0.23, Q = 36100N, d = 0.014m).
It should be noted here that the torque coefficient K has a range of ±15%, and Q=T/Kd, so if Q (axial force) becomes the minimum value, K (torque coefficient) should be the upper limit value. That is, the upper limit of the torque coefficient is K = 0.23 × 1.15 = 0.2645. Minimum torque T (Min) = 0.2645 × 36100 N × 0.014 m = 133.68 N·m.
8) Calculation of the maximum torque. The usual torque range is ±10% (20%), and the maximum torque is calculated by the minimum torque obtained above. The maximum torque T (Max) = T (Min) × 11 / 9 = 163.38 N · m.
9) Axial force maximum/minimum calculation. According to the minimum/maximum torque calculated by the formula Q=T/Kd, the axial force in the range of ±15% of the standard torque coefficient is calculated, as shown in Table 2.
Table 1 Minimum tensile load (fine thread)
Table 2 Axial force in the range of ±15% of the standard torque factor
10) Make torque / axial force diagram (as shown in Figure 3)
Figure 3 Torque / Axial Force Diagram
11) Confirm that the allowable axial force meets the maximum axial force value.
Tensile strength σ1=F/As=112000/125=896Mpa (F, As is available from the query in Table 1). The yield strength of the bolt strength of 9.8 is σ = σ1 × 0.8 = 896 × 0.8 = 716 MPa. The allowable axial force Qy = 0.7 × 716 × 125 = 62720 N according to the formula Qy = 0.7σ·As.
As can be seen from Fig. 4, the allowable axial force of the M14 bolt is 62,720 N, which is greater than the maximum axial force of 59,695 N, so it can be used.
Figure 4 allows the axial force to be compared with the required axial force
12) Benchmark analysis. Compared with the benchmark vehicle, if there is a large difference, the initial input load condition and the axial force/torque calculation process will be reviewed and reviewed.
13) CAE performance analysis. After comparison with the benchmark vehicle, if the difference is not large, the final performance verification can be performed through system analysis.
14) Picture.
5 Conclusion
The minimum/maximum torque and the minimum/maximum axial force can be determined based on the load conditions or the required axial force. Make a torque/axial force diagram, calculate the allowable axial force of the bolt to confirm whether the maximum axial force value of the torque/axial force diagram is met, and finally determine whether the selection bolt is available. At the same time, with the trend of cost reduction, the size and strength of the bolts should not be ampled. The effective bolt size and strength should be set by theoretical calculation and comparison analysis of the benchmark models.
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