Crimping is probably the most important process in wire harness manufacturing, it is the typically the sole interface between the wire carrying the signal and the where the harness connects to the next component. A single poor crimp can ruin an entire harness and a typical wiring harness could have dozens or even hundreds of crimps, any one of which could be the weak link causing a failure. This is why having perfect crimps, every single time, is the most important aspect to manufacturing a wiring harness and why you should use a profession for your wiring harnesses if at all possible.
What is a crimp joint? A crimp typically is how the connection between the wire and the terminal inside of a connector is formed. The metal terminal will have some sort of flags or crushable barrel that gets pressed onto the wire conductor to form a mechanical and electrical connection between the two. Then these terminals will typically be inserted into a plastic connector shell to complete the wiring harness assembly.
Types of Terminals. There are three popular terminal types, open barrel, closed barrel, and insulated closed barrel terminals.
Open barrel terminals are the most popular type and are usually just made from pressed metal that is formed into the shape of the terminal pin, plus the retention feature, and then the two sets of “flags” that are wrapped around the wire when crimped. These flags wrap around the wire to form an ‘M’ shape. The first set around just the bare metal wire conductor, and the second set around the wire insulation, this prevents the conductor from sliding out of the insulation if somebody pulls on the wire improperly. The shape of these crimps is vitally important and you must use a crimper designed for open barrel terminals, and preferably the exact tool or die made for the exact terminal you are crimping. This is where it gets hard for the DIY person to produce a quality crimp, because without the exact right tool, the crimp will never be as strong as it should be, and may have much higher electrical resistance as well. This high resistance can lead to heat, sparking, and arcing which can melt the connector and ruin not only the wiring harness but also whatever it was plugged into. The main advantages of open barrel terminals is that they are cheaper to produce, the crimping process can be highly automated with machines, and they are easy to inspect after the crimp to determine the crimp quality.
The next type of terminal is a closed barrel, this type of terminal has a round “tube” that the wire is inserted into , then this tube is crushed around the wire to form the crimp. Much like the open barrel terminal it is important to use the correct crimping tool to form this crimp, there are many different styles available but it’s best to use the tool recommended by the terminal manufacture. Different terminal types have different shapes they must be crimped into to hold the wire and maintain their integrity. The downside to the closed barrel style terminal is that because the wire must be inserted into the tube, it is harder to automate and high volume wiring harnesses will typically choose not to use closed barrel terminals if it can be avoided. There are some industries that require closed barreled terminals to be used however so check your specific requirements.
The final terminal type, insulated closed barrel, is typically not used with plastic connectors, but rather on single wire connectors such as spade, ring, or bullet style connections. These are exactly like the other closed barrel terminals except that they will have a crushable plastic around the barrel as well. When this type of terminal is crimped it is usually with a color coded crimp tool that matches the color of the terminal. The colors (red, blue, yellow) will correspond to the wire size that should be used as well. These types of terminals are far more universal than other terminals as far as the tools that can be used to crimp them, but they do suffer when it comes to strength and repeatability.
Quality. Crimp quality comes down to two main characteristics that can be measured. First is mechanical strength, is the wire secured to the terminal sufficiently that it can be expected that it will never loosen up or slip out? This is most often measured with a destructive “Pull test” that measures the amount of force required to break the connection between the wire and the terminal. There are a number of ratings based upon the size of the wire, the application, etc to benchmark your pull test results to. The second consideration is electrical connection. You want the electrical connection between the terminal and the wire to be as low of resistance as possible to ensure a minimum of heat transfer or signal loss between them. Milli-ohm meters can be used to measure this resistance to ensure a quality connection in a non-destructive way. Other methods of verifying a crimp quality is to cut the crimp open with a grinder and view the cross section under a microscope, called a cross section analysis. Another is to measure the crimp height and compared to a known good crimp. And finally camera systems can be used to perform visual inspections that can look for any number of faults, and measure all final dimensions of the crimp joint.