Click on the advert above to visit the company web site

Product category: Industrial Drives/Controls
News Release from: Bosch Rexroth - Hydraulics | Subject: Tower Bridge
Edited by the Engineeringtalk Editorial Team on 11 June 2002

Refurbished Tower Bridge reopens on time

When a recent engineering survey revealed problems with Tower Bridge, it was crucial that the Corporation of London took remedial action.

Britain's best known bridge is undoubtedly Tower Bridge, an internationally recognised landmark that for millions of tourists symbolises London One of five bridges owned, managed and operated by the Corporation of London, it is also the most famous example of a bascule bridge, whose central roadway span opens drawbridge-style to allow shipping to pass through

So when a recent engineering survey revealed problems that in the long term might prevent the bridge working at all, it was obviously crucial that the corporation took remedial action.

It was equally vital that Tower Bridge was closed for the shortest possible period, since tens of thousands of Londoners rely on it daily, for travelling between home and work.

The main contractor appointed to undertake this time-critical GBP 1.3 million project was Bosch Rexroth, an international market leader in drive and control technology, which replaced the original steam-powered operating system with modern electro-hydraulics in 1975, the last time Tower Bridge was closed for this type of maintenance.

Starting the initial design and preassembly work back in October 2000, Rexroth installed a system of active resting blocks, incorporating electronic load cells, under each bascule; strengthened the hydraulically driven nose bolts that interlock the two leaves; upgraded the cam-and-lever pawls at the tail end of each bridge deck; implemented a new programmable logic control (PLC) system, complete with increased monitoring equipment; and replaced the main hydraulic power units, providing 100% redundancy in the event of motor/pump failure.

The actual site work was carried out within a tight 39-day, 24/7 timescale and was completed exactly on schedule, the resulting upgrade having a life expectancy of at least another 20 years.

Designed by Sir Horace Jones and engineered by Sir John Wolfe-Barry, the construction of Tower Bridge began in 1886, because the increase in cross-Thames traffic had outstripped the capacity of existing bridges, and was finally completed in 1894.

The design needed to offer 43m clearance for tall masted ships and be shallow enough for horses pulling heavy loads, so Jones, the City architect, proposed a low level bridge on the bascule principle.

Reached by two suspension-style spans each 83m long, the main section comprises two towers, linked by an overhead walkway and a 61m long roadway beneath, constructed of two counterweighted bascules that can be raised to an angle of 86 degrees.

Although often thought to be a stone structure, it is actually a steel bridge, the twin 63m Gothic towers consisting of a steel frame clad with Cornish granite and Portland stone.

It was the largest, most sophisticated bridge ever built, driven by hydraulic power on a scale never attempted before, although early reactions to the finished structure were mixed, one trade journal describing it as "the most monstrous and preposterous architectural sham we have ever known".

Now it is one of Britain's best-loved landmarks.

Tower Bridge's two bascules or leaves, each incorporating four 49m long parallel girders and weighing more than 1000t, are moved by toothed pinion gears, which engage with steel quadrant racks riveted to the outside girders, lifting the roadway spans.

Each bascule pivots around a 25t trunnion, and the inner arm is loaded with a ballast box containing 29t of lead and 60t of iron, providing a counterbalance.

Thus the raising gear is merely required to overcome the inertia of each 31m leaf and wind pressure on the roadway.

A contemporary account of the original steam-powered operating machinery records that "the bridge actually has twice the power required, because every piece of equipment needed to lift the bascules is duplicated.

There are two sets of pumping engines, two sets of boilers and two lines of pipes leading to four sets of two lifting engines".

The boilers provided steam for the pumping engines, which maintained 750lb/in2 pressure, and when power was required, high pressure water was conveyed to the lifting machinery in the piers, each of which was equipped with three single-acting cylinders.

"When water enters the cylinders, the cylinders turn the crankshaft, the crankshaft turns the [pinion] gear wheels and the gear wheels turn the bascule drive quadrants, which lift the bascules", it concludes.

This operating system was sufficient to open the bridge for ships, then lower the bascules for the resumption of road traffic, within just five minutes, this cycle occurring over 6000 times in its first year.

Then, it provided a crossing point for 8000 horse-drawn vehicles and 60,000 pedestrians every day, although nowadays this has shifted to around 40,000 vehicles and 11,000 pedestrians daily, with only around 1000 openings a year.

During the interim the lifting mechanism remained virtually unaltered, until 1975 when the water hydraulics were replaced by modern oil hydraulics, using electric motor-driven hydraulic power packs.

Manufactured and installed by Rexroth, these units supplied power to hydraulic motors, which were connected via gearboxes to the pinion shafts and raised or lowered the bridge.

At the same time, new hydraulically operated vehicle and pedestrian gates were installed, as well as hydraulically actuated nose bolts and pawls on each bascule.

An electromechanical control system was additionally implemented, enabling the bridge to be operated from cabins on the north and south side of the river.

Problems were first experienced in the 1990s, due to an increase in vibration within the structure, particularly when heavier vehicles crossed the bridge, although a 17t weight limit was now in force.

There were also difficulties with operating the nose bolts and pawls, while the trunnion shaft bearings were showing signs of wear.

A survey was carried out by engineering consultants, High-Point Rendel, who identified that vibrations from the trunnion shaft were being transmitted directly into the steelwork of the towers.

They also discovered that the dead weight of the bascules and the live load of the traffic were not being carried fully on the resting blocks and pawls, as intended, but partly on the shaft bearings.

The pawls were not locking the tails of the bascules in the down position either and the nose bolts were not engaging correctly, due to misalignment and wear.

Since the bascules were not sitting evenly on their resting blocks, a new system of active blocks, with moving wedges and load transducers, was proposed; these, together with refurbished pawls, would allow the bascule decks to be lifted off the main trunnion bearings, as in the original design.

The ageing hydraulic system also now required all four pumps to operate the bridge at normal speed and replacement parts were not readily available, so a complete upgrade was recommended, together with a PLC-based control system, offering increased flexibility and monitoring capabilities.

"If the situation had been allowed to continue, it would have resulted in serious damage to the pivots and bearings, which would have entailed dismantling the bridge to repair", says the Corporation of London's Principal Engineer, Andrew Downes.

"Tower Bridge is a national icon and if it ceased to operate that would be a major setback.

So it was critical to complete the refurbishment work with the minimum amount of disruption, whilst ensuring it will remain operational for future use".

The contract was put out to tender by the Corporation and was gained by Bosch Rexroth, which has its UK headquarters in St Neots, in partnership with subcontractors LES Engineering and Fairfield Controls, who were responsible for the mechanical and electronic installations, respectively.

Rexroth offered the reassurance that is was technically able to handle the project and submitted a highly competitive tender.

Much work was carried out ahead of closure, including detailed site surveys and test fixings, installation of wiring and the design and build of four new hydraulic power units, complete with electric motors, pumps and condition monitoring equipment, preassembled on baseframes incorporating antivibration mounts.

There are four resting blocks on each leaf, on the river side of the trunnion or pivot, and Rexroth's scope of supply included the removal and replacement of each middle section, known as the moving wedge, so that it could be fabricated in two parts to accommodate an ABB Pressductor load cell, which feeds back data to the PLC.

If the load spread is outside normal tolerances, each moving wedge can be adjusted by means of a double-acting hydraulic cylinder (125mm bore, 300mm stroke), which incorporates an integral transducer and limit switches to determine the precise cylinder/wedge position.

For providing individual moving wedge control, there are two manifold assemblies per leaf, each fitted with duplicated Size 6/Cetop 3 Rexroth pilot-operated directional control valves, which are located adjacent to resting blocks 1 and 2 and 3 and 4.

Power to operate the cylinders on each side is provided by dedicated hydraulic power units, each with two 7.5kW motor, A10 28cm3/rev variable displacement pump sets, providing 100% redundancy as only one is required in normal use.

The nose bolt arrangement is incorporated into the leading edge of both leaves, linking the roadway span together when in the down position; the hydraulically operated bolts, with a 200mm stroke, are on the north side, the accommodating sockets on the south.

Refurbishment included the replacement of all key parts, including nose bolts, cast steel housings, sealing system and wearing pads in the sockets, machined from a self-lubricating bearing material, DEVA-Metal.

New hydraulic cylinders were also fitted, with limit switches to monitor full engagement.

The pawls lock the tails of the bascules in the down position, at the same time sharing the load with the resting blocks, and these were again refurbished in line with the enhanced specification.

Rexroth supplied custom-made cylinder actuators (160mm bore, 500mm stroke), with a cam and lock arrangement for operating the pawls, which increase the capacity for lifting the bridge deck.

There are also new rotating L-shaped footpads, complete with DEVA antifriction material, for supporting the rear of the leaves, as well as battery-powered greasers for pinpoint lubrication.

To operate the bridge, Rexroth designed and built four new hydraulic power units, each fitted with two A4 250cm3/rev variable displacement pumps and motors to give a high degree of redundancy.

Proportional control technology is employed to accelerate the bascules to maximum speed within 10 seconds, then down to creep speed prior to the stopping position.

A set of four auxiliary motor/pumpsets are also provided for operating the nose bolts, pawls and hydraulic road and pedestrian gates.

These are again based on A10 28cm3/rev variable displacement pumps, with hydraulic power control, to afford different operating speeds for the different functions.

Finally, duplicate menu-driven PLC systems with SCADA screens were installed in the control rooms on the south-west and north-east piers.

These provide pushbutton and on-screen operation of all electrohydraulic functions and a keyswitch determines what side has priority, typically the north which has additional CCTV monitoring capabilities.

Duplication also facilitates operator training, since one system mimics the other throughout all bridge processes.

Some 60 people were working on site during the closure period, operating on two 12 hour shifts, and the project was duly handed over by Rexroth, at 5pm on the scheduled completion date.

"There was a lot of prepublicity about the closure period, including advanced warning signs around the perimeter of London, so it was crucial that we reopened when we said we were going to", concludes Andrew Downes.

"We chose Rexroth because they are specialists and, at the end of the day, the project came together extremely well".

• Bosch Rexroth - Hydraulics: contact details and other news
• Email this article to a colleague
• Register for the free Engineeringtalk email newsletter
• Engineeringtalk Home Page

Search the Pro-Talk network of sites

Put your news on Engineeringtalk  |   Advertise  |   Get the free Engineeringtalk newsletter  |   Engineeringtalk Home
About Engineeringtalk  |   Copyright © 2000-2008 Pro-Talk Ltd, UK