It’s no secret the military is picky when it comes to electronics equipment. It must be capable of withstanding rugged use on land, sea and in the air. These standards encompass parts both large and small, including wire terminals for making single-wire connections to studs or terminal strips.
In fact, crimping as a means of securing wire to wire terminals and connectors was first developed in the early 1940s by the military specifically for the purpose of creating secure, rugged electrical connections. Previous connection techniques were unreliable, prone to breakdown or difficult to create.
A soldered terminal, for example, creates a rigid structure that is susceptible to cracking when subjected to prolonged vibration. Beyond that it takes a skilled worker and the right combination of solder, wire and flux to ensure the connection will be strong and electrically efficient. Excess solder can sometimes wick its way up the wire, creating a greater potential for breakage. Too much heat can damage the surrounding insulation.
With a crimped-on terminal, on the other hand, there is less danger of damaging either the wire or the insulation through incorrect installation. Although large, sophisticated, fixed crimping machines are used for high-volume applications, a handheld tool can do just as good a job for a single application-either on the shop floor, or out in the field for repairs. “All you really need is a guy with a wrench,” says Tyco Electronics (Harrisburg, PA) product manager Jim Dunbar, using a less than flattering term for the specialized crimping tools necessary to create a good attachment.
It comes as no surprise, then, that throughout the different branches of the military, crimped, solderless terminals are the rule.
This is not to say the crimping process is something that should be taken lightly. A good crimp can be as strong as the wire on which it's made. It can also be electrically "invisible" offering essentially no resistance to the current that it carries. These properties, however, are quickly lost if a crimp is substandard.
For this reason, the military is extremely demanding with regard to how exactly its contractors execute their crimps. In fact, for, say, a wire harness manufacturer looking to pick up some military work, the rigid standards may come as a bit of a shock. The U.S. government's SAE-T-7928 (formerly MIL-T-7928) specifications break the solderless wire terminal category into two subgroups: Class 1 terminals that can only be crimped with an approved tool, and Class 2 terminals that are crimped using a manufacturer's individual tooling. Class 1 dies and crimpers are defined by the MIL-C-22520 specifications. These same specifications apply to all branches of the military. Even with Class 2, the crimp must meet rigid performance requirements, generally two to three times more demanding than those required for nonaviation, civilian applications.
Beyond that, the military expects a manufacturer's crimping equipment to be correctly calibrated, since the best terminal in the world won't provide good performance if it is incorrectly attached. Some manufacturers, especially large contractors like aerospace giant Boeing, do this in-house. Others have their crimpers independently inspected by a third party to ensure that all wire-to-terminal connections will be up to spec.
"Virtually every military contract will say that you have to meet certain calibration standards," says Gary Cox, director of operations for NavCom Defense Electronics Inc. (Warner Robins, GA), a company that does diagnostic and repair work in addition to electrical assembly. He adds that for his company this means "gathering up the crimp tools into a box" every six months and sending them off for calibration, or having a private inspector do the inspection on-site for larger pieces of equipment.
"With the military you have to use the correct tool to put on a terminal," agrees Warren Smith, president of Electro-Term/Hollingsworth (Springfield, MA), a manufacturer of solderless terminals for the military. As far as Smith is concerned, though, that's a good thing. "A [solderless terminal] is not a stand-alone item. It needs to be installed correctly," he says. "Too often, people go out and buy a terminal, but they don't have the tooling to put it on. Using the wrong crimping die can severely compromise the performance of a terminal."
In fact, Smith says he is often shocked at the lackadaisical approach some manufacturers take toward the installation of solderless wire terminals, and the state of the crimps that result.
With regard to the terminals themselves, they are also submitted to regular tests as stipulated by government specifications. For Group A terminals, the manufacturer must submit the terminal to a visual and mechanical inspection, comprised of a pull test for tensile strength. Group B terminals are also tested for their electrical properties. Group C terminals must pass a battery of sophisticated tests if they are to remain on the government's qualified parts list, or QPL.
The government is currently transferring much of its specification work to the private sector. To this end, the military has farmed out the job of publishing its solderless terminal specifications to the Society of Automotive Engineers (SAE, Warrendale, PA). Actual specifications and parts numbers are unchanged, but they are now preceded by the letters SAE as opposed to MIL. The old "MIL-specs" nomenclature, however, can still be found in some catalogs.
With regard to tooling specification, the MIL-C-22520 specifications, although slated to remain firmly within the government's jurisdiction, are scheduled for an upgrade, after which they will be designated MIL-DTL-22520.
In terms of the style of connector, the military generally prefers ring-style terminals, for the obvious reason that they create a strong, secure connection. Quick disconnects, hook tongue terminals and spade styles may be convenient and desirable in the civilian world. But if you are driving a tank through the desert, your main concern is that your wire terminal remains attached.
In terms of the crimping portion of the terminal, the military generally goes with a closed-barrel configuration, as opposed to the simple, stamped open-barrel configurations common in civilian application. The reason is that closed-barrel configurations firmly grip a wire's insulation in addition to the conducting material itself. This provides increased tensile strength and vibration resistance. In many cases, tensile strength is further increased with the addition of ridges running around the inside of the barrel where it grips the stripped wire. Many stamped, open-barrel terminals include an insulation gripping portion as well. But they are simply not as rugged as closed-barrel attachments. As is the case of much of its equipment, the military prefers higher performance assemblies for reasons of durability.
The actual crimp with a noninsulated closed-barrel attachment includes a single indent in the pattern, and the crimping dies are constructed so the portion of the barrel gripping the wire is deformed differently from the portion of the barrel gripping the insulation. The entire crimp is made in one step.
In insulated terminals, like the Hollingsworth Funnel FIIG or Tyco Electronics' PIDG line, there will also be a thin copper sleeve between a shortened barrel and the insulation. This sleeve serves to grip the wire insulation, helping to secure the terminal, adding to its tensile strength. To avoid damaging the insulation, the crimp configuration used with these types of terminals does not include an indent. Smith points to the use of a noninsulated-type crimp die with an insulated terminal as a classic example of how using the wrong tooling can compromise or even destroy an installation.
Of course, using an open-barrel die, which is designed to wrap around and compress the two crimp tabs until they form a "B" is another good way to wreck an insulated, or noninsulated barrel-style solderless terminal.
With regard to the insulation itself, most military contracts call for nylon as opposed to the vinyl used in lower-grade terminals. This is because, although vinyl is a good insulator, nylon stands up better against hydrocarbons like gasoline, hydraulic fluid and oils. Teflon may be used in some high-temperature applications since it is rated for temperatures up to 260 C, while nylon and vinyl are suitable up to 105 C.
For the terminal contact itself, the military calls for tin-plated copper. Copper, of course, is an excellent conductor, while the tin provides corrosion resistance and the ability to "wipe" contaminants from the bare wire during the crimp process.
Gold plating is also used for many top-end applications, especially aerospace applications. In these cases the wire is almost always fitted with a pin-style screw-machine connector contact using an eight-indent style crimp-an attachment method that requires a special closed-frame, four-indenter-style crimp tool. In addition to creating a secure, electrically efficient connection, eight-indent crimps have the advantage of not increasing the diameter of the wire portion of the crimp. They also create a smooth, concentric attachment. The pin can then easily fit into its designated receptacle.
In the late 1960s, when four-indenter crimping machines were first being developed, they were used to create four-indent crimps. Eventually, however, designers added a notch to the indenters, which divided each of the four indents in half. This double row of four indentations or "eight" indents provides greater tensile strength, especially with smaller wires.
Putting It Together
The actual assembly process can vary dramatically, depending on the type of work being done. In fact, it can vary dramatically from job to job. Cox, for example, says NavCom has production runs that can number in the thousands or be counted on one hand, depending on the situation. Recently, for example, the company was doing upgrade work for some Special Operations aircraft. The work included many unique cable assemblies-just one per aircraft-that were terminated with simple hand tools.
Then again, when NavCom found itself manufacturing power cable assemblies for handheld global positioning system units-with orders for some 7,500 per year-it employed a semiautomatic, table-mounted pneumatic crimper to do the job.
At Heale Manufacturing (Waukesha, WI) the situation is a similar one, with production runs varying dramatically in size. Heale manufactures wiring harnesses and cable assemblies for ground-support vehicles and equipment. For larger runs, like a recent contract for wiring harnesses for armored personnel carriers, the company will have an operator run a batch of a few hundred wires through a wire-stripping machine, then go on to crimp the terminals with a semiautomatic, pneumatic crimper. Other times, it too will employ handheld crimpers.
According to Schleuniger Inc. (Manchester, NH) crimping product manager Rob Boyd these experiences are fairly typical. He says it is unusual for his company to sell a fully automated wire stripping and terminating system for defense-specific applications since the numbers generally don't merit the expense. Citing the example of a complex wiring harness assembled on a board for the aerospace sector, he notes that the operators "may only be terminating three or four wires at a time." In this situation, a simple hand crimper, or handheld pneumatic crimper would be more than sufficient, he says.
Dave Kelly, standards and industry liaison for Daniels Manufacturing Corp. (Orlando, FL), a pioneer in crimping technology for the aerospace industry, says his experience has been much the same. His company generally provides tools for fairly limited production runs-especially when compared to the automotive sector. Still, he says he is also seeing an increased demand for equipment like the Vibra-Feed automatic crimp station. The machine can create up to 1,200 eight-indent crimps per hour, compared to 300 to 400 crimps per hour on a manually operated, bench-top press.
For automatic and semiautomatic applications, wire terminals are available on tape reels. Loose terminal systems are also available with incorporated feeder bowls.