An expensive refurbishment of Tower Bridge’s hydraulic system is likely needed, according to a leading expert on the structure.
Chartered electrical engineer and author of the Haynes Tower Bridge Operations Manual John Smith told NCE that a six-figure refurb could be needed after the bridge’s twin-leaf bascules were left stuck in the upright position for more than 12 hours earlier this week.
It is the second time in the space of 12 months that the bridge’s bascules have become stuck open.
“I think that the mechanism they have in place is basically getting too old now,” Smith said. “It is more or less the same hydraulic system that was put in in the 70s. The last time I saw it, it looked pretty messy to be honest.”
He added: “It wouldn’t be cheap but I think a major refurbishment is most probably needed now which I would guess would cost at least £1M.”
While it is unclear exactly what the “technical fault” was that lead to the bridge getting stuck on Monday afternoon, Smith said that the 40+ year-old hydraulic engines which drive the shafts of each bascule are probably due for replacement soon.
The original bridge was opened in 1894 and was powered by coal-fired boilers with steam pumping engines. In 1976 the steam powered system was replaced by hydraulic rams and an electrical hardwired control system.
A new electrical control system was installed by Fairfield Control System in 2002. Some of the bridge’s mechanical items, including the nose bolts, pawls and resting blocks were also upgraded – however the system remains largely the same as that installed in the 1970s.
The hydraulic system is made up of four pumps, two for each bascule.
A description of the 2002 upgrade on Fairfield’s website adds: “Fairfields was asked to conduct a feasibility study initially to show how best to integrate a state of the art PLC/SCADA system into the existing electrical hydraulic and mechanical infrastructure.
“Fairfields proposal for a new electrical control system was accepted which incorporated a remote PLC input/output sub-system in each of the bridge machinery areas, with communication back to a Master PLC and all bridge equipment cabled back to a common point.”
It adds: “The biggest aspect of the mechanical upgrade was the introduction of ‘Active Resting Blocks’ that take the weight of the bridge off the main shaft and bearings.
“In order to achieve this, the resting blocks are automatically cycled to each bridge lift and are engaged in the down position so that the weight of each bascule is distributed evenly.
“Each resting block has load cells and duplicated analogue position transducers attached, which are used by the PLC to ensure equalised weight distribution and also for condition monitoring purposes.”
Fairfield continues to provide ongoing maintenance on the bridge’s control system.
When contacted by NCE neither Fairfield nor the City of London Corporation was able to provide further details about the reasons behind Monday’s “technical fault”.
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