The axial force state of the automobile chassis fasteners will affect the reliability and safety of the vehicle. When the axial force is insufficient, the vehicle will have major safety hazards, which will cause major market problems.

In the automobile production process, the fasteners in the chassis part are generally controlled by the torque control to achieve the axial force control. However, the torque state cannot accurately reflect the axial force distribution state of the mass production, and the axial force under the extreme bad state cannot be checked.

Based on the strain-type bolts, the paper analyzes the different structural forms of fasteners and different tools to extract the key factors affecting the axial force state, and establishes the axial force test method under limit conditions and applies it. Inspection, risk detection of extreme conditions can be achieved.

Tests have shown that the method and equipment are suitable for the risk investigation of insufficient axial force of the chassis under extreme conditions.

1. Introduction to measurement basic tools and principles

After the bolt is fastened to the chassis, the clamping force of the fastener along the central axis of the bolt is called the axial force.

This measuring bolt is made of strain-type axial force bolt. The tubular strain gauge is filled in the center of the ordinary bolt. The axial force value of the bolt is measured by measuring the strain of the strain gauge caused by the tightening of the bolt. As shown in Figure 1:

Figure 1 Axial bolt and internal structure

1.1 The principle of the axial force bolt is the Westron bridge circuit

As shown in the figure below, E is the device voltage, e is the output voltage, and R1 is the embedded strain gauge resistance, assuming initial strain R1 = R2 = R3 = R4.

Figure 2 Weston Bridge

Available

When R1 is stretched and deformed, the resistance change amount ΔR is assumed, and the formula is calculated as follows:

The actual ΔR is much smaller than R1, so:

That is, the output voltage is proportional to the change in resistance; by increasing the amplitude of the measured voltage e, a deformation amount or a data value can be obtained, and finally the strain can be measured.

1.2 Strain measurement equipment and schematic diagram are as follows

Figure 3 Axial force / torque measuring equipment and schematic

2, the discussion of the setting of the axial force limit bad detection mode

Under the specified torque, the axial force will change even if the torque is constant. In the field assembly, in addition to the monitorable factors such as the difference in accuracy (flatness, roughness, surface treatment) and the tightening torque fluctuation, the main factors affecting the axial force state of the fastener are as follows: Differences in fastener structure and equipment.

There are differences in the structure, and there is also a difference in the risk that the axial force easily reaches the limit state. Different structures are affected differently by device differences.

According to the connection structure characteristics of the chassis, two typical structures are extracted for research: single-layer connection and double-layer connection. The typical structure is shown below.

Single-layer type (most of the connection methods of the chassis, such as the fastening points of the chassis sub-frame and the body-in-white):

Figure 4 Single-layer fit structure

Double-layer (a type of connection often used in chassis suspensions, such as the lower swing arm and sub-frame point fastening points, the shock point of the shock absorber and the steering knuckle, etc.):

Figure 5 Double-layered structure

In the above two structural forms, the degree of influence of equipment differences is different; the most direct manifestation of equipment differences is the rotational speed when tightening.

The speed is set for different devices, and the same tightening torque (64Nm) is used to assemble the same fastening point. The following torque states are measured:

Figure 6 Single-layer structure measurement data

For single-layer structures, the axial force results are not significantly different.

Figure 7 Double-layer structure measurement data

The double-layer structure has obvious differences in axial force results.

From the above test data, we can get:

(1) The double-sided structure is affected more by the difference of equipment than the single-layer structure;

(2) The higher the speed of the equipment, the lower the axial force.

In summary, the conditions for establishing the axial force limit mode are: the flatness of the part, the surface treatment, the parallelism, the upper limit of the tolerance, the device speed is set as the upper limit of the parameter; at the same time, for the double-sided connection structure, such as In Figure 8, │AB│ takes the upper limit.

Figure 8 Double-layer structure control parameters

3. Example measurement description

In order to verify the effectiveness of the measurement method and system described in this paper, the author uses this method to detect a fastening point of the hem arm of a vehicle chassis.

The specified torque is 54~64Nm. The minimum detection torque is 54Nm for the limit detection setting, the flatness of the single product is 0.3% of the specified value, the coaxiality is 0.5, the upper limit is 0.3mm, and the equipment speed is set to the speed of the equipment. The upper limit is 8000r/min.

The following results were obtained. The blue point is the limit detection fastening process data, and the red point is the axial force value of the 15 measured mass production levels.

Figure 9 Limit state test results and mass production levels

graph analysis:

(1) The blue point is the data measured under the worst conditions and parts, that is, the axial force value under extreme conditions.

The final value is lower than the mass production data, which proves that the axial force under extreme conditions is indeed much lower than the normal mass production level, but the position is still greater than the required minimum axial force value of 43.75 Nm;

(2) In the worst case, it can be inferred that the axial force range corresponding to the specified torque of 54 to 64 Nm is 47.2 to 57 KN (green distribution band) and is distributed within the prescribed range of 43.75 to 68.1 KN.

It shows that even if the working condition reaches the worst case, the required axial force state can be satisfied as long as the tightening torque reaches the specified value. Therefore, the risk of insufficient axial force in the extreme case of this part can be eliminated.

From the above results analysis:

(1) The axial force in the limit detection mode will be lower than the normal mass production level;

(2) The detection method can accurately reflect the axial force value under extreme conditions and eliminate the risk of axial force limit state caused by mass production fluctuation.

4 Conclusion

Strain gauge measurement of bolt axial force is one of the commonly used methods in the inspection of automotive chassis safety parts.

Aiming at the problems of mass production fluctuation and axial force limit state risk caused by various factors in actual production, the author proposes a model based on strain gauge bolt to detect the extreme axial state.

Tests have shown that the method is easy to measure and can accurately verify the axial force of the fastener in the worst condition of the assembly, eliminating potential risks.

The measuring mode and equipment are suitable for measuring the pre-tightening force in bolts and bolt sets in the laboratory, assembly site, vibration and other environments, and provide effective measures for safety inspection and risk elimination of automobile fasteners.