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Low-voltage Wiring Harness is the network main body of special automobile circuit, the "central nerve" of automobile, the important component connecting power supply and various electrical components, and the carrier for transmitting energy and signals. The wire harness of the special vehicle frame is exposed, and the wiring loom of the whole vehicle needs to meet the technical requirements of high and low temperature resistance, salt fog resistance, chemical resistance, vibration resistance, water resistance, etc., and meet the requirements of the special vehicle driving conditions and complex environment. With the continuous improvement of special vehicles' requirements for environment, shielding and safety, the electric control system of the whole vehicle is increasing, the special automotive wiring harness is becoming more and more complex, and the failure rate of wiring harness is also increasing correspondingly, which requires improving the quality and reliability of wiring harness.
1 Overview of Low Voltage Harness
Low-voltage wiring harness refers to the wires that connect various electrical devices inside the automobile. These wires are generally bundled and exist in a bundle shape, so they are called wiring harnesses. The low-voltage wiring harness is composed of wires, terminals, connectors, corrugated pipes, heat shrinkable tubes, silicone rubber, etc. 60% of the faults of special automobile electrical system are caused by the quality of wiring harness, mainly including functional failure, appearance failure, protective failure and so on.
1) Functional failure: the problems of terminal crimping quality and terminal plugging quality directly lead to the failure of an electronic control system of the whole vehicle.
2) Appearance failure: the wiring harness is protected by open corrugated pipe and wound and fixed with electrical insulating tape, which makes the wiring harness unsightly and directly affects the appearance of the whole vehicle.
3) Protective failure: the tail end of the branch harness Connector is not protected, and the inlet end of the connector is flooded, which will lead to abnormal terminal connection.
The quality of wiring harness is directly related to the normal operation of the electrical system of special vehicles, so it is imperative to improve the manufacturing level and reliability of wiring harness.
2 Low-Voltage Wire Harness Manufacturing Process and Requirements
1) Wire cutting and wire marking (wire number): Wire cutting refers to cutting wires in batches (according to the number of models in the production plan) by a cutting and marking machine according to the wire schedule, and the cut wires are bundled and identified by labels. Wire marking (wire number) is to mark both ends of the wire with cutting and marking machine according to the wire schedule and technical requirements.
Process requirements: The cutting length of the conductor is appropriate to the length of the terminal crimping area. After cutting, there is no napping of the insulation skin, no damage or fracture of the inner core wire of the cut surface conductor, etc.
2) Wire distribution: after cutting the wires of the same wire number in batches, each wire will be grouped into a set of Wire Harness.
Process requirements: straighten out the wires of the same wire number, and then use the wire dividing tooling board to group them. The grouped wires are placed on the pay-off rack and marked, so that the wire harness is smooth, free from twisting and disorder.
3) Cut corrugated pipe, heat shrinkable pipe and heat shrinkable pipe with glue. Cut the corrugated pipe with scissors according to the specification and length cut according to the technological requirements. Heat-shrinkable tubes and heat-shrinkable tubes with glue shall be cut according to the specifications and lengths cut according to the technological requirements, and the automatic computer pipe cutter shall be used for cutting.
Process requirements: Bellows, heat shrinkable tubes and heat shrinkable tubes with glue shall be cut according to the specified size, and the cut shall be neat and smooth, without burrs or cracks.
4) Assemble the wire harness: divide the wire combination into the first branch wire harness according to the drawings and process requirements, and then wear a closed bellows (braided net) and a heat shrinkable tube with adhesive to protect the wire harness, and the branch is wound and fixed with adhesive tape; Divide each branch harness in turn, and wear a closed bellows (braided net) and a heat shrinkable tube with glue for harness protection. The branches are wound and fixed with adhesive tape first, and finally a harness combining the trunk and branches is formed.
Process requirements: the cutting size of corrugated pipe is the size required by the drawing+the length of the heat shrinkable tube at the branch/2; After the closed bellows at the branch is fixed, the size of the branch point and the size of the harness meet the requirements of the drawing.
5) Fixing and potting of branch points. After the branch points of the wire harnesses are wrapped and fixed with adhesive tape, they are protected by heat-shrinkable tube with adhesive, and the gap between the heat-shrinkable tube and each branch wire harness is filled with 703D glue.
Technological requirements: the bellows at the branch point is firmly fixed, the adhesive tape has no leakage, and the 703D adhesive has no overflow. 703D silicone rubber potting is completed in 2~3 times, the potting surface is even and smooth, and the potting height is consistent with the opening of the heat shrinkable tube, and all the gaps are potted.
6) Terminal crimping and terminal welding: Terminal crimping is the process of applying pressure to the contact area between the wire and the terminal to form it and realize tight connection. According to the terminal model, the silent terminal machine is equipped with corresponding dies and blades for crimping. The technological requirements are as follows: terminal crimping includes single wire crimping, wire crimping with sealing plug, and internal node crimping of wire harness, and the appearance quality of crimping meets the requirements of QC/T29106-2014 Technical Conditions for Automobile Wire Harness. Terminal welding is the welding between aviation connector welding cup and conductor core, which is manually welded with electric soldering iron.
The technological requirements are as follows: the solder joints have good wetting appearance, good surface gloss, no wrong welding, false welding, missing welding, no mechanical damage, etc. After self-checking that the welding position of each wire is correct and firm, push the heat-shrinkable tube to the bottom of each welding cup for heat-shrinkable protection; GD-414 silicone rubber is used for potting, and the potting height is more than 2mm without soldering cup. The inspection of terminal crimping and welding process is as follows: the crimping and welding quality of each wire terminal shall be visually inspected, the self-inspection process shall be recorded by multimedia, and the inspector shall inspect it. Only after the inspection is qualified can the next work be carried out.
7) Assembling connector: the wiring harness connector is mainly composed of pins, shell, tail clip, sealing plug, secondary lock and so on. Connectors are divided into male connectors and female connectors. Insert the terminal into the corresponding connector jack according to the insertion point required by the drawing, install blind plug at the vacant position according to the technical requirements, and seal GD-414 silicone rubber at the inlet end of the waterproof connector.
Process requirements: when inserting terminals, pay attention to the direction of terminal insertion, and the hook side of the terminal is in the same direction as the internal limit end of the connector. The terminal should be inserted straight to avoid the terminal being skewed. The terminal should be inserted into the wire inlet hole of the connector until a "click" sound is heard. Gently pull the wire to check whether the terminal is properly locked. Self-check is carried out by adopting the principle of "push-pull push". When a clear sound is heard, it means that the plug terminal has been locked by the internal buckle of the connector. Pull the wire by hand, and then push the terminal forward after the plug terminal does not pull out.
GD-414 silicone rubber is encapsulated at the inlet end of the waterproof connector, with no holes or spots, even and smooth encapsulating surface and no overflow at the front end of the plug.
8) Heat-shrinkable tube: Before assembling the connector, thread the adhesive heat-shrinkable tube into the tail of the harness. After the connector is assembled, the heat-shrinkable tube fixes the corrugated pipe and the tail of the connector, without exposed wires, and plays a waterproof role.
Process requirements: when baking the heat shrinkable tube with glue, the electric heating equipment should be adjusted to 250 ~ 350℃, and the processed product should be heated evenly and moved slowly, and it is not allowed to stay in one place for more than 10s; when baking the heat shrinkable tube without glue, the hot air gun should be adjusted to 150 ~ 250℃, and the processed product should be heated evenly and moved slowly. It is not allowed to stay in one place for more than 2s. After the heat shrinkage is finished, the surface of the heat shrinkable tube is clean, flat, free from damage, cracks, wrinkles, bulging and edge warping at both ends. When one end is pulled by hand, there is no displacement, detachment and exposed terminal crimping part.
9) Making labels: sticking PET self-adhesive name labels on the tail end of the harness; After the assembly is packaged, stick paper labels on plastic bags and cartons; Two kinds of labels are printed by a printer and then pasted.
Technical requirements: when pasting, the wide edge of the label is parallel to the harness, and the label is free from skew. The edges of overlapping labels are aligned, and the labels are flat. Paste the action once, and avoid uncovering and re-pasting the wrong position found in the paste process. The label has a smooth surface, without wrinkles, warping and other phenomena.
10) Harness conduction detection: mass-produced vehicle models make wiring harness conduction tooling, and connect the conduction instrument to conduct conduction detection; Conduction test is carried out with multimeter for small batch production vehicles.
Process requirements: each harness assembly must be tested for harness continuity, and the points of each wire are correct, without short circuit, open circuit, virtual connection, etc.
11) Inspection application: detailed introduction is as follows.
First inspection: before crimping various types of terminals, respectively, perform tensile test on the terminals, and report to the inspection department for inspection. Truthfully fill in the tensile test record form in duplicate, signed by the inspector, and report to the inspection department for one copy, while the team keeps one copy.
Self-inspection: perform self-inspection after each process of wire harness production, and fill in the self-inspection form truthfully.
3. Inspection: video multimedia recording shall be made in the self-inspection process of terminal crimping, terminal welding, terminal plugging and contact connection. Operators in the same process shall conduct other inspections, and inspectors shall conduct random inspection of multimedia records.
Full inspection: the appearance, size and conduction detection of each set of harness assembly shall be fully inspected by the inspector.
Process requirements: strengthen the inspection of wiring harness manufacturing process, strictly implement the inspection process for each wiring harness assembly, and fill in the inspection records truthfully.
12) Package protection of each branch wiring harness end: after the wiring harness assembly passes the inspection, each branch wiring harness end (connector) is protected by plastic bag and fixed with rubber band.
Process requirements: during the process of Wire Harness Manufacturing, every procedure should be well controlled to keep hands, operating platform, operated equipment and tools clean, so as to avoid the excess from entering and affecting the performance of wire harness. After the wiring harness is finished, the wiring harness connector shall be protected.
13) Harness packing: Pack the qualified harness assemblies into cartons or plastic bags, and affix labels to indicate the harness drawing number, name, batch number and quantity.
Process requirements: the harness assembly shall be packaged and marked.
3 Low-voltage wire harness manufacturing process improvement
The process improvement of wire harness is comprehensively improved from the aspects of improving the appearance quality of wire harness, saving the manufacturing cost, upgrading the equipment, perfecting the process flow and method, etc., so as to make a high-standard new wire harness that meets the technical requirements of special vehicles.
3.1 Selection and Promotion of Harness Connector
Wiring harness connector is the core component of wiring harness, and its performance directly determines the overall performance of wiring harness, and plays a decisive role in the stability and safety of electrical components of the whole vehicle. The connectors are waterproof connectors with waterproof sealing rings, and meet the technical requirements of waterproof level. The connector is provided with a terminal secondary locking structure and a molded case secondary locking mechanism, which ensures that after the connector is assembled, the connection is firm and the terminal does not retreat. After investigation, AMP and Dechi waterproof connectors are generally used for special automobile connectors.
3.2 Improve the Protection Level of Harness.
Investigate the protective components that meet the environmental requirements of special vehicles and their installation process. The harness is upgraded from open corrugated pipe protection to closed corrugated pipe protection, and it is protected by heat shrinkage with adhesive heat shrinkable tube. The closed bellows is closed as a whole except for the open port. Compared with the open bellows, the threading process is complicated, and water is easy to enter at the branch point.
After investigation and test, the closed bellows threading adopts the circulating process flow of separating the branch wiring harness, threading the bellows of the main and branch wiring harnesses, and fixing the branch points of the bellows. In order to improve the waterproof performance of the wire harness, the branch points of the wire harness are fixed by adhesive tape, heat-shrinkable tube, and silicone rubber potting. XX heat-shrinkable tube is selected for heat-shrinkable tube, and XX silicone rubber is selected for silicone rubber. Both of them have passed the tests of high and low temperature, waterproof and salt spray. When the branch point is fixed, the front trunk and the branch harness are fixed in the center by tape winding, and the whole winding method is adopted to wind the branch point of the harness (the position of the branch point required by the drawing), and the winding length shall not exceed the length of the heat shrinkable tube with adhesive; After the bellows is firmly fixed, center the rubber-coated heat-shrinkable tube which is worn in advance at the heat-shrinkable branch point, and the adhesive tape has no leakage. Silicone rubber is filled in the gap between the heat shrinkable tube and the bellows of each branch harness, and the surface is cured within 24h.
3.3 Improve the Appearance Quality of Wire Harness.
The tail clamp of the end connector of the branch wiring harness and the bellows port are protected by thermal shrinkage with adhesive thermal shrinkage tube instead of electrical insulation tape, which is beautiful in appearance, sealed and waterproof.
After technological analysis, pay attention to the following points during operation.
1) The length of the heat shrinkable tube should be tightly fixed at the tail of the connector/aviation connector and the tail of the bellows, and should not be too long to avoid affecting the installation of the wiring harness.
2) When the connector with the buckle is thermally shrunk, the thermal shrinkage of the heat-shrinkable tube cannot exceed the buckle, so that the buckle cannot be used after being fixed by thermal shrinkage.
3) After confirming that the specification and size of the heat shrinkable tube are correct, wear the sleeve in advance.
4) Bare conductor protection: single conductor is thermally shrunk with non-adhesive heat shrinkable tube, and corrugated pipe tail and harness tail are thermally shrunk with adhesive heat shrinkable tube.
3.4 Fully Consider the Actual Situation of Wiring Harness Assembly
After investigation and analysis, the harness in the cab is protected by woven net, which solves the problems of small space in the instrument panel and difficulty in assembling the harness.
Fixing the braided net at the branch: thread the braided net into the main harness, then thread the braided net with corresponding specification into each branch harness to the branch point, and then fix the braided net end of each harness with adhesive tape (full winding). Fold the braided net opening of the main harness inward by 50mm, completely wrap the braided net with the fixing tape, then fix the folded part (point wrapping) with the tape, and then tie it with a cable tie. After fixing, the size of the branch point meets the requirements of the drawing.
3.5 Select Suitable Advanced Equipment.
Select advanced equipment suitable for the production of whole vehicle wiring harnesses of special vehicles, meet the mass production of whole vehicle wiring harnesses, and customize non-standard equipment. For example, if the all-in-one wire stripping and marking machine is used, the printing of wire number is changed from ink spraying to laser marking, and one device completes three processes of wire cutting, wire stripping and wire marking at the same time, which saves the consumables for wire harness production and improves the efficiency of wire harness production.
Ensure the accuracy and quality uniformity of terminal crimping in the key process of wire harness manufacturing, break the generalization of dies, customize different dies and blades according to the terminal type, and install them in each terminal machine by category, and select different specifications of terminal machines for crimping according to the wire diameter.
3.6 Facilitate the Electrical Debugging and Maintenance of the Whole Vehicle.
In order to facilitate the electrical debugging and maintenance of the whole vehicle, name labels are attached to the ends of each branch harness. After investigation and test, PET self-adhesive labels meet the requirements of high and low temperature resistance, salt fog resistance, chemical corrosion resistance and water resistance. Print the batch number and name in the yellow area of PET self-adhesive transparent label. The batch number is pasted on the end of the trunk line harness of each harness assembly, which is convenient for quality tracing.
4 Harness Manufacturing Quality Control
1) The whole wiring harness manufacturing process adopts the fixed number of employees and posts, and the assembly line operation. Set up a separate wiring harness team, and the staff will take up their posts after theoretical and practical training in each process, and will receive regular process training to improve the quality awareness of operators.
2) Work out the operation instructions and on-site process documents for each process. The operation instructions include the equipment, process flow, process requirements, precautions, etc. used in this process. First of all, the operation instructions are placed at each station on the production site, so that operators can grasp the key points of the process at any time. Secondly, the wiring harness drawings of specific vehicles are decomposed, and on-site process documents such as cutting schedule of each conductor, name label printing table, corrugated pipe cutting schedule and the like are compiled, so that operators can see at a glance, which greatly improves the production efficiency and reduces the operation error rate.
3) According to the model of the parts to be operated, select the equipment and molds with corresponding specifications; according to the equipment maintenance manuals, regularly maintain the equipment; Before operation, first check the integrity of the equipment, such as whether the blade is worn or damaged, whether the mold is firmly installed, etc. During the wire harness manufacturing process, strengthen the control of surplus materials, and keep the equipment, platform and tools clean and tidy; Turn off the power when the equipment is used.
4) In the process of wire harness manufacturing, the terminal, connector, bellows and other parts used should be self-checked to ensure that there are no quality problems such as cracks, corrosion and damage before operation.
5) Ensure the quality of key processes of wire harness production, and strengthen inspection links. Wire harness is the key part of special vehicle. In the manufacturing process, each process must be inspected according to the inspection process.
5 Concluding Remarks
Low-voltage wiring harness is the blood vessel and nerve of the electrical system of special vehicles. This paper of Kable-X introduces the manufacturing process and requirements of low-voltage wiring harness for special vehicles, including not only the selection of wiring harness connectors, the improvement of the appearance quality of wiring harness, the improvement of equipment and the improvement of technological process, but also the quality control of wiring harness manufacturing. Generally speaking, the manufacture of low-voltage wiring harness for special vehicles has great development prospect and room for improvement, and further discussion and research are needed to better ensure the reliability, stability and safety of the electrical system of the whole vehicle.
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