Threaded fastener connection is one of the important connection methods in the assembly process of the whole vehicle and has been widely used.

The tightening process may seem simple, but the tightening of the different materials and the different tightening methods will have different effects on the fastening effect.

In the process of vehicle quality inspection, the static torque monitoring of bolt fasteners is an important part. Whether the static torque is qualified directly affects the assembly quality and the quality of the whole vehicle.

Therefore, through the calculation of the static torque control range, the empirical calculation method of the static torque control range is summarized and applied to the actual torque monitoring to verify the feasibility and accuracy of the method.

1. Dynamic and static torque definition and measurement methods

An external force applied to the bolt retaining member first overcomes the frictional force acting on the fastener to enable the bolt retaining member to rotate in the direction of tightening.

The bolt fasteners have only two states of rotation and static, thus dividing the bolt fastener torque into dynamic torque and static torque.

The dynamic torque is the range of torque control that is designed to calculate the axial preload required for the fastening of the part and requires the tightening process to be performed.

The assembly site is tightened using a median set of tightening tools for dynamic torque. The torque peak obtained during the tightening or torsion process is the dynamic torque measurement. Dynamic torque is used to tighten process control.

The static torque is the torque that continues to tighten the threaded fastener in the tightened state and the relative rotation of the thread pair.

When measuring the static torque, use the torque measuring tool to gradually increase the torque of the tightened bolt along the tightening direction until the bolt reaches the "turn-to-turn" error, and the torque value of the newly generated motion recorded by the electronic instrument or the dial pointer. , the torque value is the static torque measurement.

Static torque is used to monitor process I. The static torque control range can be determined by the acquisition of static torque during actual production.

2. Static torque control range calculation method

According to GB/T26547-2011 "Test method for performance of rigid-turning tools for threaded fasteners", the threaded connection state can be divided into soft connection, hard link and neutral connection.

The static torque control range of the six bolts at the hinge of the door is neutral, and torque attenuation and backlash are less likely to occur after tightening. The bolt distribution at the hinge of the door is shown in Figure 1.

Figure 1 Bolt section at the door hinge

Static torque collection was performed using a CEM100N3X15DG digital torque wrench, as shown in Figure 2. When in use, put on a sleeve of corresponding size, set the corresponding data acquisition mode, and collect static torque according to the operating specifications.

Figure 2CEM100N3X15DG digital display torque wrench

2.1 Data collection

The bolts 1 to 4 at the door hinge are M8 mounting bolts, and the bolts 5 and 6 are M10 mounting bolts;

The dynamic torque of the bolts 1 to 4 released by the design engineer is (32~2) N·m, and the dynamic torque of the bolts 5 and 6 is (55~3) N·m;

In the production stage, the process engineer uses the secondary tightening method to set the tightening torques to 32 N·m and 55 N·m, respectively, and tighten the above six fastening points;

Then, within 5 min, the quality engineer used the CEM100N3X15D-G digital display torque wrench to measure the static torque, and recorded 30 measurement data of each bolt separately. The recorded results are shown in Table 1.

Table 1 Static torque measurement value of bolt

Continued Table 1

2.2 Data Analysis and Processing

Taking bolt 1 as an example, the average static torque is calculated to be 34.89 N·m, and the standard deviation (Sigma) is 2.69. According to the 3 Sigma principle, the upper limit of static torque is 42.96 N·m; the lower limit is 26.82 N· m.

The upper and lower limits are averaged and the static torque control range is 27~43 N·m, which is (35~8) N·m. For the same reason, the static torque control range of other bolts is calculated, as shown in Table 2.

Table 2 Calculation results of bolt static torque related parameters

2.3 Process stability and judgment of the validity of the results

For the collected static torque data, the stability of the tightening process needs to be analyzed. Figure 3 shows the XbarR diagram of the bolt 1. It can be seen that the measurement data is stable and controlled without abnormalities.

Figure 3 Xbar-R control chart for bolt 1

The calculation result of the static torque needs to be judged for effectiveness, and the nominal value of the static torque needs to be satisfied not to exceed the nominal value of the dynamic torque of l5%. For bolts 1 to 4: (35/32 - 1) × 100% = 9.4% < 15%;

For bolts 5 to 6: (61/55 - 1) × 100% = 10.9% < 15%. Therefore, the control range of the static torque is effective.

After calculating the static torque range, the process engineer feeds back to the product engineer, who combines the torque sampling data of the new product trial and road test stages to determine the static torque value.

Combined with the characteristics of the neutral connection of the door bolt thread pair, it is finally determined that the static torque control range of the bolts 1 to 4 is 27 to 43 N·m; the static torque control range of the bolts 5 to 6 is 47 to 75 N·m.

Before the formal production, the process engineer will release the standard of all dynamic and static torque of the whole vehicle to the manufacturing department and the quality department. The manufacturing department performs bolt tightening based on dynamic torque, and the quality department performs inspection and monitoring based on static torque.

3. Empirical calculation of static torque range

As can be seen from the static torque values, the static torque values ​​are mostly outside the range of dynamic torque.

Because the torque required during the static torque measurement process is greater than the tightening torque, there is a measurement deviation, so the static torque needs to be increased based on the dynamic torque.

At the same time, if the static torque range is calculated according to the above method, the workload is large. Therefore, on this basis, based on the production experience and the dynamic torque released by the product engineer, an empirical calculation method of static torque is summarized.

3.1 Determine the static torque intermediate value based on the measurement deviation

After a large number of data acquisition calculations, the dynamic torque of bolts 1 to 4 is 32 N·m, the static torque is 35 N·m; the dynamic torque of bolts 5 to 6 is 55 N·m, and the static torque is 61 N·m.

For the characteristics of the neutral connection of the bolt thread pair, it can be seen that the increase of 10% based on the dynamic torque can be approximately defined as the intermediate value of the static torque.

3.2 Determine the range of the inspection torque based on the measurement error

The relationship between dynamic and static torque tolerance values ​​and measurement error is shown in Figure 4. a is the tolerance value of dynamic torque; b is the tolerance value of static torque; c is the 3 Sigma value of measurement error and static torque measurement error.

It can be seen that the relationship between a, b and c satisfies the mouth + c = 6.

Figure 4 Relationship between dynamic and static torque tolerance values ​​and measurement error

Taking bolt 1 and bolt 5 as an example, the static torque is the measured value, and the dynamic torque is the set value. The difference between the two is the measurement error.

Therefore, the distribution of measurement error is also the distribution of static torque and dynamic torque difference. The dynamic torque, static torque and the difference between the two are shown in Table 3.

Table 3 Correlation Torque Values ​​for Bolts 1 and 5

Continued Table 3

From Table 3, it can be calculated that the Sigma values ​​of the measurement error distribution of the bolt 1 and the bolt 5 are 2.66 and 4.61, respectively, and the corresponding 3 Sigma values ​​of the bolts 1 and 5 are 7.98 and 13.83, respectively.

Substituting the relationship, the static torque tolerance values ​​of bolts 1 and 5, ie b values, are 8.22 N·m and 14.15 N·m, respectively, and the static torque tolerances are 8 N·m and 14 N·m, respectively. The results obtained are in agreement with the results of the second part calculation.

3.3 Empirical calculation method of static torque

The 3 Sigma values ​​of the distribution of the measurement errors of the bolts 1 to 6 are 7.98, 7.95, 7.96, 7.98, 13.83 and 13.80, respectively, and the values ​​are about 25% of the respective dynamic torques.

Therefore, an empirical algorithm for measuring errors, that is, a neutral connection to the door bolt fastening, is proposed, and the measurement error is generally ±25% of the intermediate value of the dynamic torque. For the neutral connection of the door bolt thread pair, an empirical algorithm for static torque control is proposed:

1) The intermediate value of static torque is increased by about 10% based on the intermediate value of dynamic torque;

2) The measurement error is about 25% of the dynamic torque intermediate value;

3) According to the relationship between measurement error, dynamic torque tolerance and static torque tolerance, the tolerance of static torque is calculated, that is, the control range of static torque is determined.

4 Conclusion

Bolt fasteners are the most widely used connection method in the assembly process of automobiles. The control of their torque is an important guarantee for the quality of the whole vehicle.

At present, the concept of dynamic and static torque has been used in the actual production of domestic automobile manufacturers.

Combined with the calculation method of static torque and the actual situation of torque monitoring in the production process, an empirical static torque calculation method is proposed. The method is verified by a large number of subsequent torque tests, and the feasibility is verified.

At the same time, the method can be applied to the whole vehicle to assemble other bolt thread pair neutral connection, and the production efficiency can be improved under the premise of ensuring the manufacturing quality of the whole vehicle.