2003-02-27 - London’s Tower Bridge has been given a new lease of life following the installation of a load monitoring system by ABB Limited. Eight ABB Field IT Millmate Pressductor® load cells form the heart of a new load monitoring system that ensures weight is evenly distributed across the bridge.
The raising and lowering of the bridge decks is controlled by separate hydraulically powered gear mechanisms housed in the towers at each end of the bridge. To ensure that the decks remain aligned when lowered, the load across the bridge decks needs to be equal.
When closed, the two main bridge deck structures each sit on four resting blocks. The dramatic increase in the weight and volume of traffic over the bridge has worn the main bearings on which the deck structures pivot. This led to an imbalanced load across the bridge, with some bearings taking more load than others. If left unresolved, this would have caused further damage to the bearings and pivot mechanisms and would have resulted in the bridge having to be dismantled for repair.
To overcome the problem, the old stationary resting blocks were replaced with active blocks jointly designed by ABB Limited and LES Engineering of Grimsby, under the control of the principal contractor, Bosch Rexroth Ltd, and the Corporation’s Engineers High Point Rendel.
ABB FieldIT Millmate Pressductor load cells housed in each of the new blocks enable constant measurement of the load across the bridge decks. Capable of measuring loads up to 600 tonnes, the cells relay load data for each deck via a 4-20mA signal to a programmable logic control system regulating the position of the resting blocks. If the signal shows an imbalance, the position of the resting blocks can be adjusted by hydraulic cylinders to maintain an equal load across the bridge.
Installed in January 2002 as part of a major maintenance programme, the new active resting blocks will help to both increase the life of the bearings and the overall bridge structure.
"The original designers of the bridge are believed to have wanted to develop a way of transferring all the load to the resting blocks when the bridge was built in 1894, but were unable to do so at the time," says Greg Stratford, Engineering Director, LES Engineering. "By working with ABB, we have made this a reality and have come up with a solution that should ensure the bridge can continue to function without a problem."
ABB’s Millmate Pressductor load cells have been widely used in many applications since they were first developed in 1953. Common applications include monitoring of hot and cold rolling in the steel industry, as well as measurement of tension and flatness, together with use in offshore applications.