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Wire harness adapter is used to connect the connecting points of automobile circuit, it is an important part of automobile, its quality directly affects the transmission effect of power or signal. The plug-in occupies a small proportion of the cost in the automobile, but if the quality problem occurs in the use of the automobile, it will often have serious consequences, and the maintenance cost will increase greatly. Therefore, the quality of the plug-in has been paid more and more attention by the automobile manufacturers and parts suppliers.

Terminal withdrawal is a common failure mode of wire harness connectors. Terminal unclipping means that after the connector is assembled, the terminal is improperly separated from the sheath, resulting in the loss of the connector function. The causes of this failure generally include three aspects: ① the holding force of the terminal in the sheath is not qualified; (2) interference with insertion; ③ Product application problem. According to these three reasons, this paper analyzes the terminal withdrawal of connector and puts forward the solution.

The holding force of the terminal in the sheath is unqualified

The holding force of the terminal in the sheath is the force required to separate the terminal from the sheath along the axis.

In order to avoid the phenomenon of terminal withdrawal, the performance test standard of the connector has strict regulations on the holding force of the terminal in the sheath: ① The holding force of the connector whose size is not greater than 2.8 is greater than 40N; ② The holding force of a connector larger than 2.8 is greater than 60N.

In practice, many terminal withdrawal phenomena are caused by the holding force does not meet the requirements. Generally speaking, the reasons for the unqualified holding force of the terminal in the sheath are: structural design problems and material selection problems.

1.1 Structural design problems

The structural design problem mainly refers to the connection structure design of terminal and sheath. The hanging structure is generally an elastic structure, which is divided into elastic structure on the jacket and elastic structure on the terminal.

1.1.1 Elastic structure is adopted on the sheath

The elastic structure is adopted on the sheath, and the elastic structure required by the assembly of the terminal and the sheath is designed on the sheath. During the assembly of the terminal and the sheath, the elastic tongue (the elastic structure designed on the sheath) is stressed and deformed to realize the assembly of the terminal and the sheath. After the terminal and sheath assembly are in place, the sheath tongue returns to the original state due to the release of the force. The combination of the jacket spring tongue and the hanger structure designed on the terminal ensures the effective positioning of the jacket to the terminal. The shear strength of the sheath tongue determines the holding force of the terminal in the sheath. The shear stress is calculated as follows:

Where: τ -- shear stress of the material; F -- force in shear direction of material; A -- shear area; [τ] -- allowable yield shear stress of materials.

According to formula (1), the maximum force under shear direction is proportional to the shear area of the material and to the allowable shear stress of the material. In the design of sheath tongue, when the material is selected, the allowable yield shear stress of the material is fixed. To ensure that the product meets the requirements of the retaining force (shear direction force) of the terminal in the sheath, it is necessary to ensure that the shear area of the sheath tongue meets the required value.

In practice, considering the design gap between the terminal and the sheath, the shear area should be calculated according to the extreme case. The design gap between the terminal and the sheath shall ensure that the yield shear stress that the calculated shear area can withstand under extreme severe conditions is greater than the required holding force of the terminal in the sheath.

1.1.2 Elastic structure on the terminal

The elastic structure on the terminal means that the elastic structure required by the terminal and sheath assembly is designed on the terminal. In the process of the terminal and sheath assembly, the terminal and sheath assembly is realized through the stress deformation of the bullet tongue (the elastic structure designed on the terminal). After the terminal and sheath assembly are in place, the end bullet tongue returns to its original state due to the release of force. The combination of the bullet tongue and the lanyard structure designed on the jacket ensures the effective positioning of the jacket to the terminal.

The yield strength of the bullet tongue determines the holding force of the terminal in the sheath. According to formula (1), in the design of end bullet tongue, when the material is selected, the allowable shear stress of the material is fixed. To ensure that the product meets the requirements of the retaining force (shear direction force) of the terminal in the sheath, it is necessary to ensure that the shear area of end bullet tongue meets the required value.

In practice, due to the consideration of the design gap between the terminal and the jacket, in addition to considering the shear area (material thickness and the width of the tongue) of the end bullet tongue, it should be ensured that the end bullet tongue and the hanger structure designed on the jacket have a complete fit under extreme conditions.

1.2 Material selection

1.2.1 Sheath material selection

Connector sheath materials commonly used are: polyamide (commonly known as nylon), polybutanediol terephthalate (PBT), ABS, etc.

For the sheath elastic-tongue structure, the toughness of the material is generally considered, and the size of elastic-tongue structure is designed according to the allowable yield shear stress of the material meeting the requirements of toughness.

It should be emphasized that the allowable yield shear stress of the material should be calculated according to the allowable value after the material is subjected to the possible high temperature aging, temperature-humidity cycle, chemical liquid corrosion and other tests of the connector. If the elastomer structure designed according to the allowable yield shear stress value after the test cannot meet the requirements, it should be considered to replace the material or use the secondary lock structure (FIG. 2) to solve the problem.

When the initial holding force of the connector is normal, but the holding force is unqualified after the test and the needle is withdrawn, it is generally caused by the elastic tongue structure designed according to the initial allowable yield shear stress of the used material.

1.2.2 Terminal Material Selection

Connector terminal commonly used materials: red copper, brass, bronze. According to their hardness can be divided into soft, semi-hard, hard 3 states.

Among these three materials, copper is less used, generally used for ground hole or fork joints. Brass and bronze are often used. Brass conducts electricity better than bronze, and bronze has better hardness and elasticity than brass. When end-bullet tongue construction is used, bronze should generally be used. Brass materials often have insufficient retention.

2 Terminal interference

Terminal plug interference refers to the failure mode that the male terminal is inserted into the invalid area of the female terminal when the male terminal is inserted into the female terminal, resulting in loss of functions, such as terminal withdrawal. Generally speaking, the reasons for terminal to plug interference are as follows: ① structural design problems; ② Terminal plugging force problem.

2.1 Structural design problems

The structural design problems of connector involving terminal to plug interference mainly include: center distance design problems of connector (including size error or large tolerance), clearance design problems between terminal and sheath (including too large or too small clearance).

When there is a problem in the structural design, the male terminal and the female terminal will be misaligned during the plugging process of the connector. When this misalignment occurs in the x direction (FIG. 3), the male terminal will be on the top of surface B or surface C. If the misalignment occurs in a minor degree, the inserting and pulling force of the connector will be too large, and if the degree is serious, the terminal will slip out during the plugging process of the connector. When such misalignment occurs in the y direction (FIG. 4), the female terminal will be exposed unreasonably, and the male terminal will be easily stuck in the female terminal exposed. If the degree of misalignment is mild, the inserting and pulling force of the connector will be too large, and if the degree of misalignment is serious, the terminal will slip out during the plugging process of the connector.

For example, during the assembly process between the fuse box and wire harness in the engine room of the assembly workshop of an automobile factory, the terminal of the connector at the end of the wire harness frequently receding and the insertion force was large. The rate of needle receding was 15%, and the proportion of large insertion force was all. Through the analysis of the problem parts, it was found that due to the design of the center distance of the fuse box structure, the male terminal and the female terminal were not in alignment during the fuse box insertion process, which resulted in the insertion force of the wire harness terminal connector and the fuse box insertion process and the part of the wire harness terminal connector was out of alignment. The problem is solved by improving the structure of the fuse box.

2.2 Terminal Plugging Force Problem

Due to the requirements of assembly performance, the smaller the plug and pull force of the connector terminal, the better. Too small plug and pull force will increase the contact resistance of the connector and reduce the conductivity of the connector. Therefore, the appropriate plug and pull force of the terminal can meet the requirements of the assembly performance of the connector on the premise of ensuring the electrical performance of the connector. In order to improve the electrical conductivity of the terminal, the plug and pull force of the terminal is often increased in practice, which generally leads to assembly difficulties. If the terminal insertion and removal force is greater than the holding force of the terminal in the sheath, the terminal may slip out during insertion.

3. Product application problems

It relates to the product application problem of terminal withdrawal of connector, mainly refers to the problem that may cause terminal withdrawal in the process of wire harness production. Generally, the problems include terminal crimping and terminal assembly.

Banana terminal is a common problem during terminal crimping. Refers to the excessive bending of the terminal due to improper crimping, so that the center line of the junction area and the crimping area of the terminal are not parallel, as shown in Figure 5.

When the degree of excessive bending is relatively light, the inserting and pulling force of the connector will increase and the assembly performance will be affected. If the severity is severe, the male terminal cannot be inserted into the valid insertion area of the female terminal. As a result, the terminal slips out.

For example, during the assembly of the rear taillight and wire harness in the assembly workshop of an automobile factory, the terminal of the wire harness end connector was frequently clipped, with a rate of 20%. Through the analysis of the problem, it was found that some terminals were excessively bent (banana terminal) due to improper crimping of the terminals, which resulted in the clipping of the terminal of the wire harness end connector during the assembly of the rear taillight. After adjusting the crimp of the terminal, the problem was solved.

4 Conclusion

There are three reasons for the pin withdrawal of connector terminals:

(1) The holding force of the terminal in the sheath is not qualified, which is generally caused by design reasons, manufacturing reasons or the use of materials;

(2) Insertion interference -- generally caused by design reasons or manufacturing reasons;

③ Product application problems -- mainly caused by inappropriate use methods in the process of product use.

In actual use, if the terminal needle withdrawal situation, you should first analyze the cause of the problem, to solve the problem.