The promise of high-definition television (HDTV) is great, with dramatically improved picture and sound quality in the wide-screen, digital signal. But its adoption has been slowed by the need to convert nearly every piece of hardware in the industry to a new broadcast infrastructure, including cameras, transmitters, and telecommunication lines.
Even the towers that transmit HDTV signals must be far taller. The largest can be 2,000 ft tall, with guy wires as big as 1.5 inches in diameter attached at various heights to protect against wind and weather.
And that's where Preformed Line Products (PLP) comes in—the company makes anchors for the guy wires that support the towers, as well electrical conductor and optical fiber cables, splice cases, and cross-connects. Their newest anchor is called the Rocket-Socket Dead-End, and it looks like a two-foot long, 60-lb tuning fork. It needs that great size to support enormous stresses; with structural loads up to 252,000 lbs and even greater loads possible from accidental collisions.
Each guy wire terminates in a cone-shaped wedge that fits inside the dead-end housing. Until recently, PLP dead-ends were made of ductile iron, and attached to galvanized-coated, steel-based strands up to 11/4 inches in diameter. But the latest cables are larger, from 15/16 to 17/16 inches.
So PLP engineers thought about using a heat-treated material, austempered ductile iron (ADI), to make dead-ends stronger without increasing their size. It's more expensive to produce, thus often used in brake calipers and other critical components. But since this was a new application, PLP had to test the performance, geometry, and cold-sensitivity of the new design.
They chose ALGOR software to test the static and dynamic forces on their Pro/ENGINEER CAD models: "We first did a linear static stress analysis with ALGOR's FEA software and it looked good, especially considering that we have a high safety factor built into the Rocket-Socket," says PLP project engineer Phil Pisczak. "But we were also concerned about dynamic stresses. One possible source is normally wind loading that causes wires to vibrate; but based on years of field experience, we knew we had a strong product capable of withstanding this loading."
Rather, the stress he was most concerned about was abuse in the field—since the dead-end is shaped like a tuning fork, it vibrates and produces high resonance when struck. That can boost stress from 2 to 10 times what the part would usually experience. But impact analysis in the lab was a challenge, since their normal tools—accelerometers and cameras—weren't up to the job.
"ALGOR's MES (mechanical event simulation) software allows us to look at what is happening in an impact event without high-speed cameras or complicated laboratory tests," he says. "MES is our microscope into the dynamic impact process. We get to see the motion, dynamic loading, and stresses. Those results tell us much of what we could learn with the most sophisticated lab tests."
Pisczak simulated 12 mph and 48 mph drop-test impacts, and watched the resonance waves travel up and down the part. He noticed that while the magnitude of stress was well within the material's strength, it peaked at a certain feature—the fillet at the base on one of the "ears." So he changed the housing design, using a larger-radius fillet for the physical prototype. In the mandatory, physical lab test, the new design stood up to tensile bed stretching, and then 20 five-foot drops at room temperature, and five more at -40F.
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