The manufacturers of a complicated multi-section wiring harness will start with the drawings, which detail the construction, as well as specifying each required part and section of the design. A computer-aided assembly engineer will draw the wire harness design; decide the wire type, conductor size and strand count, as well as the insulative coating and color of each wires type. They will specify the length of each of the many various arm sections, the various connectors required, and the type of cable covering. To ensure long life in use and proper strain relief for each connector, the final environmental insulative and protective wrap type will be specified. If overmolding with protective thermoplastic elastomer (TPE/TPV) molding compounds are required, they will be specified, and a drawing supplied at this time. The machining department will mill out and manufacture the multipart injection molds for the large plastic injection machines. They test the mold design with the proper connector and wire harness to ensure proper fill and complete encapsulation of the part. Special water spray and thermal testing can be done on the test overmold part to ensure compliance is within design requirements.
After the cable manufacturing company receives the drawing and possibly a R&D sample, a manufacturing methodology is decided. A complicated wiring harness can involve many wires routed along different paths with different connectors. A router board is designed with wire retaining clips at each bend and connector. The correct wires are precut to length on an automatic machine. Usually, each wire is a different color so the operators can quickly and correctly follow the color path drawn on the router board in order to lay the wires in the correct order. The operator strings each wire by color and type along the route laid out on the assembly board, ensuring that the ends match the stop point for connectors. Once all the wires are in place on the router board retainers, the operators apply either temporary tape, pull ties, permanent self-adhesive tape or heat shrink which is heated to secure the wires in place. Now that the first stage is completed and each wire secured, the strung wiring harness can be removed from the router board to send to the next stage.
The next stage ensures that the ends of the wire are automatically trimmed to the correct length and stripped of the insulation to the correct length. The bare wire ends are usually lead-free solder tinned or left bare for copper crimping. If heat shrink insulative protective coating is required at the connector end, it can be applied and shrunk in place at this point before the connectors are soldered to the wires.
The connectors are attached to the wires by various methods; usually either crimped on with special machines that ensure proper closure around the wire or they are soldered. After all the connectors are attached and the solder joints or crimping is inspected by microscope, the final outer coating and connector strain relief is applied. Many connectors have small lengths of heat shrink tubing fed on to the many connectors’ wire before the connector is soldered. After soldering inspection, the heat shrink tube is slid over the connector joint and heated to shrink in place, providing addition insulation between connector posts as well as increasing the strain and stress relief between the wire harness and the connector.
Sometimes, it is required by the design that that overmolding with rubberized compounds or plastic be molded around the connector to encapsulate the wire harness, creating a water-resistant connection with added strain relief at the connector. When all the connectors are soldered on the wire harness, it is fully inspected. The semi-completed wire harness is sent for electrical testing. An automatic machine will be utilized for this. All connectors are plugged into the engineered test frame and the assembly will be automatically checked for continuity of all connections. Then, it will perform high voltage testing of insulation of wire and connector dielectrics, verifying connector insulation. A final outer protective coating can be wrapped on at this time, although sometimes it is done on the router board.
Many times, an R&D sample is manufactured for further endurance testing, from saltwater resistance to pull strain relief and additional tests such as strength pull tests between connectors and the wire harness, as well as bending of the joint between specified connectors and the wire harness body. Utilizing an automatic testing machine bends the assembly through a specified radius a required number of times, checking flexing continuity during each bend cycle.
The final test is a quality control for each completed wire harness, measuring each section for the correct length and construction of the wiring harness, verifying the proper connectors and the final over molding or protective coating quality. The finished product can have protective caps put on connectors, if so specified, carefully packaged, and shipped to the customer.
Cooler Cao is an engineering manager at Cabletree Industrial Company. Cables, cable assemblies and wire harness are, more than ever, an integral part of today's electronics. With the development of automotive and medical diagnostics, electric automobiles, solar and wind power has come the design and develop of newer and more sophisticated connection devices.