By automation or manually by hand, Interpower® crimps bridges for country-specific plugs, and IEC 60320 connectors onto miles of flexible cable it manufactures each year. For cord sets, the optimal crimp is one that provides the most electrical contact between the conductors of stranded wire and its bridge or connector—the B crimp provides optimal electrical contact for flexible cable.
Interpower North American and international cable, power cords, and cord sets are manufactured in Iowa. These cords and cable are comprised of insulated conductors braided into a “length-of-lay” (each conductor with the same degree of twist measured one full turn lengthwise along the cord’s axis (diameter) via the cabling machine’s rotating capstan. Afterward, the three braided conductors are ready to be respooled and sent down the extrusion line for jacketing. This greatly increases the flexibility and strength of the jacketed cord, such as Interpower’s three-conductor NEMA 5-15 and 5-20 standard or hospital-grade cords and cord sets. Lay length also makes spooling cable easier.
In the U.S., the measuring tradition is to produce cable in American Wire Gauge (AWG) in even numbers, e.g., 12, 14, 16, 18—the larger the gauge, the smaller the cable. The number of strands inside a conductor isn’t selected randomly but by design. If running 18 AWG cable comprised of three-conductor stranded wire, the conductors, after being cut and crimped, must mirror the cross-section, the circular mil area (CMA), of one solid 18 AWG wire. The CMA is calculated by converting its 18 AWG diameter into a square-inch figure (0.0403 which converts to 0.001276 sq. in.). The circular mil area of a circle has a diameter value of 0.001—so how many of those 0.001 circles fit into a 0.0403 circle? The CMA of 18 AWG, then, is derived by dividing 0.001276 by 0.000000785, which puts the CMA at 1625. The CMA of the strands of wire, whether 7, 16, 19, 41 or 65 strands (for 18 AWG), must replicate the CMA of a solid wire within 98 to 100% accuracy.
The fewest number of strands for 18 AWG is 7, with 6 strands surrounding the center strand (nucleus) in a hexagonal shape despite its round appearance. Seven-strand wire contains 6 interior voids (spaces) and 6 perimetric voids, i.e., 6 spaces around the nucleus strand, and 6 spaces outside the perimeter strands. Another stranding combination for 18 AWG is using 19 strands, which contains 24 interior voids and 12 perimetric voids. All strand centers lie on a hexagon. Controlling the compression of conductors is a basic B crimp tenet.
The first challenge for crimpers begins upon receiving the stripped wire (conductors)—the cut wires have expanded from the cutting, which reduces the amount of strands that will insert into the crimp barrel. Despite wrestling with expansion, the B crimp can still provide a perfect honeycomblike crimp for flexible power cords. At the top of the wire roll tool is the outline of a “B" as if it had been knocked over to the left, its two curves sticking straight up. The wire roll tool lowers and gathers the wire into the U-shaped terminal and is compressed with a preset (correct) force in the anvil so that the terminal’s wings fold inward to create the B shape, but without cracking the crimp or otherwise misshaping it through not enough or too much force. Now, honeycomblike, the CMA’s surface should mirror a solid conductor’s surface as to provide an optimal electrical pathway. The difference is what was once a “line” contact is now a “solid” contact after producing a good B crimp.
Parts of the Crimping Machine
Laboratory test results reveal the hexagon crimp design (the shape of the overall crimp, not the compressed polygons of the B crimp), raise terminal temperatures in flexible cable upwards of 170°C, compared to the B crimp’s comparatively low 115°C, which is notable since it is not desirable when terminal temperatures rise above cable temperatures (120°C.) That the B crimp keeps its cool while the hexagon crimp raises terminal temperatures above cable temps makes the B crimp the better choice of the two for flexible cable, especially where fire safety is concerned. However, the hexagon crimp performs admirably with rigid cable, with cable and terminal temps testing out at 120°C and 110°C respectively.
The B crimp is no stranger to manufacturing—its crimp on wire is stronger than ever after its initial success in the 1950s. Prior to the 1950s, wire terminations were typically soldered. It should be noted that the Interpower lab in Ames, Iowa, at the direction of Product Development Manager, Ron Barnett, mirrors UL labs in many regards, and concerning crimps, conducts a crimp cross-section test where the crimp barrels (the wings) on each side are cut in half before they’re polished and washed in acid to produce an extremely smooth finish. Then, the crimped section is photographed under a microscope. The images are then viewed on a computer monitor.
Magnified Image of Crimp Cross-Cection on a Computer ScreenAlso, the Interpower lab conducts a temp-rise test. At the Temp-Rise Station, thermocouples attached to components of electrical contacts measure temperature rise. The thermocouples are soldered onto the blades of the plug or connector as to monitor temperature when connected to a power source, especially its rate of rise. By UL 817 standards, if it rises less than 30°C, the test is successful (North America). For International Standards (IEC 60320 and IEC 60884), that margin of error is 45°C. A reading is taken every minute as a software program charts the rise in real time. If the temperature is found to rise too quickly, subsequent testing ensues to discover the faulty component(s).
Certainly, other tests at Interpower’s lab take their shots at crimp strength (such as the UL 817 Abrupt Pull Test), and those can be found in the Product Design Library menu tab, above. But are all crimp test results so objective?
According to John D. Butler, a noted director of engineering and technical writer, the best of crimp images can be deciphered differently: “Part of my time was spent discussing image quality with a number of radiologists around the country. I became aware of the limitations of human visual discrimination and acuity regarding image quality. Even to highly skilled doctors, there are times when one pair of eyes will detect something in an image that another may not . . . granted, the decisions made on the magnified images of terminal or connector crimp cross-sections are rarely involved in life or death situations. However, the goal toward achieving the best image possible should still be the same.”
Butler, of course, is referring to all types of imaging, how it often gets interpreted differently—from X-rays to crimp images on a computer monitor. Since Butler’s analysis, HD resolution has made light-year jumps. Yet, no matter how advanced the technology, human decision-making as to what the images actually depict has the final word.
Special thanks to John D. Butler, PE, and Interpower Vice President of Manufacturing and Logistics, Mike Boyle, and Interpower Product Development Manager, Ron Barnett.
Read more about crimping in upcoming InfoPower and Connections articles.