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Space and weight dictate mobile bridge launcher drives

21 November 2012

The importance of mobile bridges has grown as military forces and rescue organisations alike seek to extend the usability of heavy equipment by using transporters to bring them closer to the front line before they are deployed. This poses problems; for example, the combined weight of a battle and its transporter is around 96 tons, quite beyond the capacity of most mobile bridge systems.

There is one, however, that was designed specifically for this purpose - the WFEL dry support bridge (DSB), which is essentially the culmination of more than 40 years experience gained by WFEL in the construction of military mobile bridge systems.

A DSB’s launch vehicle uses compact, lightweight planetary gearboxes and winches to reduce overall weight and achieve a higher power density. It can launch a DSB to span gaps of up to 46m and be deployed in under 90 minutes. Indeed, WFEL’s ability to design long distance spanning, fast deployment bridges has not gone unnoticed around the world and its bridges are in service by some 39 armed forces across the globe.

The company recently won a contract from the US Department of Defense (DoD) to supply the fourth phase of a framework agreement, originally agreed in 2000, for a new batch of DSBs. To date, the US has ordered almost 100 of these systems, worth in excess of $450m. WFEL, which earlier this year unveiled a £200,000 investment programme at its Stockport manufacturing facilities, remains one of the few prime contractors to the US DoD which is wholly UK based.

The large distances over which the DBS units have to be transported, plus the usually harsh and uncertain nature of the terrain, means that they not only have to be robust but also as compact and lightweight as possible. WFEL’s development design for a wheeled deployment vehicle addresses these requirements in two ways: by using aluminium as the primary material of construction and by employing components that provide high levels of output and reliability from the smallest possible operating envelope. In terms of gearboxes and winches this means planetary units.

These were employed after experience with the prototype launcher vehicle showed that conventional gearboxes and winches were too heavy for the output power required. Brevini planetary units were chosen to provide the same or greater levels of torque output as the original conventional gearboxes but at around half their size and weight. As a result, two multistage gearboxes, from Brevini’s standard range, and two specially adapted variable speed winches are used on each vehicle.

The function of the gearboxes is to drive the telescopic launching beam, which is mounted on the back of the vehicle. The launching beam is extended progressively, under the low speed control provided by the combination of the gearboxes and hydraulic motors, until it reaches the extent of the span. The bridge sections are then attached to carriages on each side of the beam and indexed into position by the variable speed hoists (where Brevini units are also used). The total process has been designed to be deployed in under 90 minutes and is completely interlocked to prevent any inadvertent operations, which could damage the structure.

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