Machinedesign 17666 Lift Bridge Promo G2

Lift Bridge Redesign Relies on 21st Century Cables and Cable Carriers

Oct. 2, 2018
A cable management system replaces drop cables and improves reliability while reducing maintenance.

After 95 years in service, a lifting bridge connecting New Hampshire and Maine needed to be torn down. The one that replaced it might look like the nearly century-old bridge, but it is equipped with some revolutionary technology, including plastic cable carriers from igus (which it calls energy chains). They carry and protect the cables carrying power and data for the motors and sensors involved in lifting and lowering the bridge.

They also improve reliability and dramatically reduce maintenance costs by preventing the cables from rubbing against each other and creating wear, or swinging uncontrollably in the weather. The motor’s power and control cables, as well as the fiber-optic cables, are also from igus.

The rugged carriers and continuous-flex cables were installed inside the lift mechanism to maximize reliability and uptime, while reducing costs when compared to previous, less-dependable alternatives—mainly droop cables. Droop cables, typically used in similar applications such as elevators, are typically very expensive custom cables, which combine several cables into a single jacket to provide all power and data.

Today’s Memorial Bridge runs along the East Coast of the U.S. across the Piscataqua River. It replaced the original bridge of 1923. It is equipped with a vertical lift that raises the bridge to let ships sail under it. The bridge must rise more than 128 ft at an average rate of about 4ft/sec.

As they are custom-built, these cables also tend to have extensive lead times, which can back up project deadlines and run costs up further. Right after installation, droop cable requires extensive testing; many bridge proposals outline the full test procedure as a part of the initial agreement, including who must be present as a test witness. Then, once in service, the repeated lifting motion lead to frequent breaks and damage to the cables.

To avoid these problems, the project team decided to use a variation of igus’ E4 energy chain series that could handle the harsh Northern New England weather and wind, as well as moisture, UV, and chemical exposure. The carriers were pre-harnessed with guaranteed igus’ chainflex continuous-flex cables with specialized CFXL strain-relief clamps, staggered for easier access while in the final long, hanging orientation. Each cable installed, included motor and power cables, along with 4-core fiber-optic data cables which were extensively batch-tested in real-world application settings over billions of flexing cycles prior to installation.

Motor cables, heavy-duty CF3000 CF9, were enclosed in thermoplastic elastomers (TPSs) and resisted hydrolysis, microbes, UV, oil, and fire. The metal-free cables for fiber optics, CFLG, are protected by a polyurethane other jacket, remain flexible in temperatures down to −40 F°, and resist UV and oil.

The lifting bridge uses pre-assembled igus’ readychain cables and cable carriers instead of the original hanging cables. (The orange lines show where one of the flexible cable carrier is mounted behind a major support beam.) They offer a much higher level of safety and reliability combined with reduced maintenance.

The igus team worked directly and closely with the bridge’s design and maintenance teams, creating a custom metal trough to house the cable carrier system, blending igus design principles with the bridge itself. A combination of structural steel plates, some more than 10 mm thick, were used to create the trough, which was designed to accommodate wind loading, sea, salt, and UV exposure, as well as bridge design safety standards and customer requests.

The system was, on request, designed for a 50-year lifespan. A custom weather-seal brush system was also customized and installed to keep icing to a minimum and prevent any freezing or hindrance of free movement of the chain carrier.

The new cable management systems can be seen in operation in the video clip below.

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