Signalling Equipment at Tower Bridge
by John Hinson & Phil Everett
John Batts kindly provided three photographs of the signalling equipment at Tower Bridge prior to conversion to electric operation.
“There were I think 2 cabins, one at the foot of each tower downstream side. Maybe some interlocked lookout upstream also. Hydraulic power for the bridge operation was generated by steam engines under the roadway on the south side (one still on show) & stored in a big hydraulic accumulator which is still adjacent. Power for lifts was taken from the London Hydraulic Power Co. mains & either could be used as a standby for the other. Now it’s all electric & LHPC mains carry fibre optic cables. Sad.”
It is thought that this first picture shows the control levers in the north-east tower. The 1976-installed electric controls have replaced these levers and these work the bridge today:
This picture depicts, it is thought, the controls in the south-east tower. These levers remain but are no longer operational:
Each frame operated a bascule independently. The levers, manufactured by Saxby and Farmer, are interlocked by what looks like a conventional tray under the operating floor, and worked the bridge by means of hydraulic valves. The levers had functions like the selection of the hydraulic engine to be used, the removal of the pawls that support the bridge deck when it is open to road traffic (the deck is lifted slightly to take the weight off the bearings), and a brake. (One may also have worked the central bolts that kept the lowered bascules together).
There were four hydraulic engines per bascule, two each side of the road, one large, one small, selected by lever. The two capstan-like handles in the upper photo admitted the water from the hydraulic accumulators (two each side of the bridge) to whichever engine was selected.
In the upper photo, to the left of the left capstan, is the lever (in a quadrant) and the rod that worked the signal in the lower photo (or its counterpart). It has been moved to the museum. The lever in the south-east cabin is still there, but not its signal.
In both photos, visible between the capstans in the upper photo and above the man’s right elbow in the lower, is a smaller elevated Saxby & Farmer lever frame which worked the road and river signals hydraulically. The lever plates are still there in the south-east cabin and can just be read after years of polishing. They are interlocked by a small vertically mounted locking tray, and seem also to have been linked to the bridge mechanism so they could not be moved until the bridge was right. It seems they superseded the manual signal, as in the museum there is a centrally pivoted lower-quadrant railway-like signal that is hydraulically actuated that was said to be used to signal to river traffic until the 1950s.
The equipment was not worn out when it was replaced in 1976, but the reduction in river traffic and the infrequent use made the continued use of steam boilers uneconomic. It is now powered by electrically driven hydraulics.
Hydraulically worked river signal that replaced the original semaphore signals in the 1950s: