Type: Center Bearing Swing Span
Location: Amesbury, MA
Client: Massachusetts Department of Transportation
Stafford Bandlow Engineering personnel were retained by the prime contractor on this design build project to resolve various mechanical and electrical issues that developed during construction and commissioning of this new Swing Span over Merrimack River in Amesbury, MA. SBE involvement included resolution of various mechanical and electrical issues in coordination and cooperation with the prime contractor and the Engineer of Record.
Mechanical issues included corrections to the span drive machinery alignment, changes to the span balance and changes to the elevation of the balance wheel track. SBE provided personnel to determine the root cause of the problems and then directed survey, millwright and field machining operations to produce satisfactory machinery alignment, proper span balance and a level balance wheel track. In addition, SBE conducted strain gage testing of the span drive machinery to determine system friction and to determine load sharing between the two independent electro-mechanical drives.
SBE provided specialist electrical control and drive staff to analyze and resolve bridge control logic and drive operational issues. The bridge drive system consists of two duty drive motors that are independently controlled and powered from variable frequency drives. The drives were intended to share bridge operating load equally but were failing to do so and the imbalance between drives was a cause of overloading of one drive at the expense of the other. Additionally, it was reported that both drives were experiencing overload conditions which worsened with decrease in temperature. Following an extensive analysis and electrical load testing of the drive system, SBE determined that the required load sharing algorithm between the drives had been incorrectly set up, speed control feedback to the drives was incorrectly connected, drives had not been tuned or matched to their respective drive motors and the mechanical drive system was imposing excessive operating loads on the drives. SBE, in conjunction with the drive manufacturer, corrected each of these issues which resulted in reliable operation of the electrical drive system under all loading conditions.