Crimping Technology of Automobile High Voltage Harness

Starting from the performance requirements and operating characteristics of automobile high-voltage Wire Harness, this paper analyzes the main factors (terminal structure, crimping mode, crimping height and crimping length) that affect the crimping quality of cable and Connector terminals in the crimping process, and puts forward some suggestions for optimization design. Through the relevant test of the crimping performance of automobile high-voltage wiring harness bundles, it is verified that the segmented crimping makes the automobile high-voltage Cable Harness bundles have better mechanical properties than the whole-segment crimping. The better mechanical and electrical properties of automobile high-voltage wiring loom bundles can be ensured by reasonably controlling the crimping height and length by using the crimping method of confining pressure and spot pressure.

1 Connection Mode of Automobile High-Voltage Wire Harness

Generally, there are three kinds of connection modes between automotive Wiring Harness cable and connector terminals: welding, crimping and mechanical connection. The welding method mainly adopts soldering connection, which is suitable for automotive wiring harness connection and assembly with small batch and small cable cross-sectional area; The crimping method mainly applies external force to the corresponding specification die to combine the connector terminal with the cable contact surface, which has reliable connection and long service life. It is suitable for mass processing, but it needs the crimping die to realize the connection and assembly. The mechanical connection mode is mainly to tightly combine the connector terminal with the cable through fasteners, and the assembly is relatively complicated, so it is not suitable for mass production.

At present, overseas automotive wiring harness cables and connector terminals are generally connected by crimping, and the connection technology is very stable and reliable, while the production in automotive wiring harness in my country is still in the development stage, especially the reliable connection technology in automotive wiring harness has not been fully mastered. According to my long-term experience in automotive wiring harness connection and assembly in Kable-X, among the three connection methods of welding, crimping and mechanical connection, crimping is the most applicable, and the connection is fast, reliable, firm and long-lived, which is suitable for mass processing. Therefore, it is most important to study the crimping technology for the reliable connection between automobile high-tension cable and connector terminals.

2 Crimping Process of Automobile High-Voltage Wire Bundle

The rated current transmitted by automobile high-voltage wire harness is large, up to several hundred amps, so the selected cable has a relatively large diameter, which puts forward higher requirements for the crimping quality between the cable and the connector terminal. In order to ensure the high quality of the cable and connector terminals after crimping, and to ensure that the electrical properties (such as rated current) and mechanical properties (such as pull-off force) of the automobile high-voltage wire bundle after crimping meet the requirements, the following analysis is made on the main factors (including terminal structure, crimping mode, crimping height and crimping length) that affect the crimping quality of the cable and connector terminals in the crimping process.

2.1 Terminal Structure and Crimping Mode

The terminal structure can be divided into contact section, middle section and crimping section. The contact section is used to ensure connector docking, electrical power and signal transmission; The middle section is the receiving area between the contact section and the crimping section, which ensures that the contact section and itself do not deform in the crimping process, and plays a positioning role at the same time. Once deformation occurs in the crimping process, it will seriously affect the performance of the automotive wiring harness. The crimping section is used to connect the connector terminal with the contact surface of the cable under external force, and its quality directly affects the conductivity, pull-off force and appearance of the automotive wiring harness.

When the traditional closed-tube terminal is crimped with the cable, it is formed by one-time crimping. And the whole model can be simplified into a simply supported beam. Although the pressure F on the whole terminal crimping section remains basically the same, the moment M (= FL) is also different (M1 > M2 > M3) because of the difference in the length L of the moment arm (taking the stepped surface as the fulcrum) (L1 > L2 > L3). According to the stress theory of simply supported beam, the bending moment is the largest at the L/2 of the crimping section, which leads to the difference of the crimping height (that is, the cross-sectional height of the crimped part between the connector terminal and the cable terminal after crimping), so that the real contact area after crimping is only a small part, which is only equivalent to line contact. This has seriously affected the electrical and mechanical properties of automobile high-voltage wire bundles with large cable cross-sectional area, high tensile performance and long crimping length (that is, the contact length between connector terminals and cables after crimping).

In order to ensure the electrical and mechanical properties of automobile high-voltage wire bundle after crimping, and avoid too small contact area caused by different crimping heights in one-time crimping molding process, after optimized design, the connector terminal of automobile high-voltage wire bundle adopts segmented terminal. Segmented terminals and cables are crimped by segment crimping molding, which can reduce the original crimping length, save the design space and meet the requirements of the pull-off force and conductivity of automobile high-voltage wire bundle by crimping two sections successively.

The finite element method is used to simulate and analyze the displacement of the traditional closed-tube terminal crimping section and the optimized segmented terminal crimping section when the crimping section exerts the same force. It can be seen that after crimping with the same external force, the crimping section of the traditional closed-tube terminal is bow-shaped, with the maximum displacement at 1/2 near the end face, which is the crimping contact surface between the cable and the contact terminal. The optimized segmented terminal crimping section is drum-like, with the largest displacement in two places and relatively small deformation in the middle step of the two crimping sections. In the simulation crimping process, the cable has a larger contact area with the terminal, and the crimping section with high middle and low two places forms a barb shape, which strengthens the connection between the cable and the contact terminal.

2.2 Crimping Height and Crimping Length

In order to ensure the electrical and mechanical properties of automobile high-voltage wire bundle after crimping, besides reasonable terminal structure and crimping method, the crimping height and crimping length of contact terminals should also be ensured in the actual crimping process.

If the crimping height is too high, there will be too large invalid gap in the crimping area, resulting in insufficient contact area between the cable and the metal conductor of the connector terminal, which can not meet the crimping strength (i.e. the retention force between the terminal and the cable), pull-off force and conductivity required by the automobile high-voltage wire harness, and even lead to abnormal working state of the crimping terminal; If the crimping height is too low, it is easy to break the cable core or the metal conductor in the crimping area, which does not meet the requirements of automotive wiring harness crimping. Therefore, the crimping height between the cable and the connector terminal must be strictly controlled. Point pressure, confining pressure and other methods are often used to crimp connectors. Generally, the crimping depth of point pressing is D/2 (D is the outer diameter of the terminal). At this time, although all the gaps between the cable and the terminal can be compressed, the pressure pit is too deep, which will easily lead to the cable core being deformed too much and pressed into a sharp corner, resulting in the tip effect of electric field. In severe cases, the cable core will even be broken, resulting in the deterioration of the electrical continuity and conductivity of the automobile high-tension cable bundle. Generally, the crimping depth of confining pressure is D/3. At this time, although the compression deformation is relatively uniform, the outer layer of the cable copper wire core deforms first when it is compressed, while the inner layer is basically not stressed, which often leads to the phenomenon that the outer layer is tight and the inner layer is loose, which has a certain influence on its electrical conductivity.

In view of the shortcomings of spot pressure method and confining pressure method, it is suggested to adopt the combination of confining pressure and spot pressure to control the crimping depth to 0.4d after optimization design, so as to effectively compress the terminals and cables. If the crimping length is too long, it will easily lead to excessive crimping force and waste of materials, resulting in low structural utilization rate of the crimping area; If the crimping length is too short, the contact area between the terminal and the cable will be too small, which can not meet the crimping strength (i.e. the retention force between the terminal and the cable) required by the automobile high-voltage wire harness, and at the same time, the conductivity will be too low. Therefore, the crimping length between the cable and the connector terminal must be strictly controlled.

Where: FT is the pull-off force of the corresponding terminal, that is, the pull-off force of cables of different sizes (standard requirements are shown in Table 1); FZ is the friction force on the contact surface between the terminal and the cable; R is the radius of the cable after crimping.

Table 1 Standard Requirements for Pull-off Force of Cables of Different Sizes

2.3 Crimping Performance Test

In order to further study the influence of terminal structure, crimping mode, crimping height and crimping length on the electrical and mechanical properties of automobile high-voltage wire bundles after crimping, taking automotive wiring harness with rated current of 200 A (the selected cable cross-sectional area is 25 mm², and the maximum passing current is 300 A) as an example, an experimental study on the crimping performance of automobile high-voltage wire bundles was carried out. As shown in Table 2, the crimping process of each automobile high-voltage wire harness sample in the crimping performance test of automobile high-voltage wire harness is shown in Table 2, in which sample 1 adopts the traditional crimping process, sample 2 adopts the optimized terminal structure, crimping mode, crimping length and traditional crimping height, and sample 3 adopts the optimized terminal structure, crimping mode, crimping height and crimping length. It can be seen that the electrical and mechanical properties of sample 3 are the best. This shows that the optimized crimping process can ensure the high quality and high performance of automobile high-voltage wire bundle after crimping.

Table 2 Crimping Process Technology Used for Each Automobile High-Voltage Wire Harness Sample

3 Conclusion

Starting with the performance requirements and usage characteristics of automobile high-voltage wire harness, this paper analyzes the main factors (terminal structure, crimping mode, crimping height, crimping length) that affect the crimping quality of cable and connector terminals in the crimping process, and puts forward some suggestions for optimization design. Through the relevant test of the crimping performance of automobile high-voltage wire bundles, it is verified that the segmented crimping makes the automobile high-voltage wire bundles have better mechanical properties than the whole-segment crimping, and the better mechanical and electrical properties of automobile high-voltage wire bundles can be ensured by reasonably controlling the crimping height and length with the combination of confining pressure and spot pressure.