High Flex Cable for Bridges
High Flex Cable Lifts Bridge Performance With DuPont™ Hytrel®
When the high flex cable of a busy navigation bridge failed , Northwire Inc. was called to engineer a solution. Their design used high-flex cable insulated with DuPont™ Hytrel® thermoplastic polyester elastomer to restore the busy link between Minnesota and Wisconsin, and keep it in service.
The 140 foot-vertical span of the Stillwater Bridge, connecting Minnesota with Wisconsin, is raised and lowered at least 20 times a day from May through October during navigation season, using cable that is unwound and rewound as many times. And with 10,000 to 20,000 cars passing over the bridge each day, the Stillwater is an important driving link for the region. So, when the bridge’s cable failed after just two years, engineers called Northwire, Inc., a leading maker of insulated cables, to design and manufacture an industrial-grade high-flex cable.
Northwire created a high-flex-life, multi-conductor cable insulated with Hytrel® thermoplastic elastomer. Because Hytrel® provides effective electrical insulation and toughness in thin layers, the replacement cables are over 40% smaller in diameter than the cable that failed. The prior cable was insulated with ethylene-propylene rubber to encase its 50 conductors (16 AWG) and measured 1.86 inches . The Hytrel® replacement is just 1.1 inches and its conductors are each insulated with 0.010 inch (0.25 mm) of Hytrel®.
“Smaller cable diameter means longer flex life at a given bend radius,” explains Ted Beach, sales director at Northwire. Simply stated, the thinner cable winds and rewinds more easily. Hytrel® is a good choice for cables that flex because its combination of high dielectric strength and toughness allow thinner layers than many alternatives. Adding to the project’s success was the quick turnaround schedule, with design, approval and production taking just three weeks, as opposed to several months for the original cable.
In the case of the Stillwater Bridge service restoration, DuPont™ Hytrel® was chosen for its high performance over traditional rubber insulation, and its demonstrated effectiveness with:
- Better winding flexibility.
- Reliable performance.
- Thermal and weather resistance.
Northwire has built on the success of the insulated cable made with Hytrel® by obtaining UL recognition, allowing it to be used for robotic and industrial automation equipment applications. Northwire’s cables for robotics typically use a 0.008 or 0.010 inch layer of insulation. And although PVC can also provide adequate insulation, its mechanical strength doesn’t compare to Hytrel®.