United States Department of Transportation
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Edition:Final Report, February 2022 – August 2024
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Abstract:A potential vulnerability of a tower drive vertical lift bridge is a failure to maintain level operation over the length or width of its movable span; this is known as longitudinal or transverse skew, respectively. When either of these conditions occurs, they can cause the movable span to jam in its guides; without adequate protection, this can lead to a catastrophic bridge failure. Vertical lift tower drive skew indication and monitoring has historically used a differential synchro (i.e., differential Selsyn) system. However, this technology is now considered obsolete, and replacements are difficult to obtain. A study was commissioned to evaluate alternatives to the differential Selsyn system. These alternatives were selected based on several criteria, including their availability, ease of maintenance and replacement, and minimization of the use of advanced electronic equipment. This report summarizes Phase 2 of the study. In this phase, several alternative skew detection methods were field-tested and evaluated. Based on the results of this evaluation, an alternative to the differential Selsyn system was selected to provide the required skew monitoring, indication, and operating system protection. The considered alternative systems included the use of direct skew measurement using an inclinometer and indirect skew measurement using encoders. To minimize maintenance and mean-time-to-repair, as well as limit dependency on PLC systems, consideration for the deployment of these alternative methods included the use of SMART relays with self-diagnostics that can be replaced easily in the event of a malfunction.
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