How does Tower Bridge work?

Ask anyone to name one fact about Tower Bridge, and the majority will answer that the road opens up to let boats through. But have you ever wondered how Tower Bridge works?

It is often assumed that Tower Bridge is a drawbridge, as the way the road opens up looks very similar to what you would expect to see on a medieval castle. This was actually how Tower Bridge’s architect, Sir Horace Jones, initially intended the roads to open and was the design submitted to the Tower Bridge design competition. However, it was discovered that the roads would be too heavy for the towers to hold, so a new solution was needed.

A bascule bridge

The answer was to make Tower Bridge a bascule bridge. ‘Bascule’ is a French word, which can be translated as seesaw, and describes how the two sides of the road open. Each bascule moves around an off-centre pivot, meaning they look a bit like a seesaw in a children’s playground. Moreover, each side of the bascule is evenly balanced, so less energy is required to open up the roads and no pressure is put on the Towers.

The hard work of opening and closing the Bridge is left to eight large cogs. These are about 1m in diameter, four on each side, which rotate to open and close Tower Bridge.

The power required to rotate the cogs was initially supplied by steam and then, post-1976, by electricity.

Explore the Bascule Chambers

Lifting Tower Bridge then: steam engines

Steam was the energy of choice during most of the 1800s. The London Hydraulic Power Company powered much of London, including cranes, lifts, workshops, and even theatre machinery. At Tower Bridge, power was generated by three large boilers, continuously fuelled by stokers working in shifts, 24 hours a day, to ensure there was enough energy available to lift the bridge whenever required. The steam created in these boilers was used to power the large steam pumping engines in the next room.

These engines are brightly painted and resemble a steam train when they move. The engine’s pistons turn a large flywheel at the same time as powering a set of hydraulic pumps, which pushes water into six huge storage containers called accumulators. They act like batteries, storing the energy by holding the water under high pressure.

Then, when the Bridge was required to open, the water was released into the drive engines, powering yet more pistons to turn a set of cogs. These cogs connect to a rack on the back of the bascules, opening them for passing river vessels.

Cogs are still used today to open Tower Bridge, but the power is generated in a different way.

How does Tower Bridge open now?

When Tower Bridge was first built, London was an extremely busy trading port. However, as trading ships became larger, fewer and fewer vessels made the journey to the Upper Pool of London, which goes from London Bridge to the Cherry Garden Pier, in Bermondsey. A much larger port was then built further down the Thames, and closer to the sea.

By the late 1960s, Tower Bridge only opened a few hundred times a year (down from over 6,000 in the first year), and steam was no longer an efficient way to open it. So, in 1970, it was decided that something needed to be done to make it more efficient and economical to lift, reducing its emissions and bringing it into line with new rules around clean air in London.

The installation of a new electro-hydraulic system began in 1974. Electric pumps were used to create oil-hydraulic energy to turn the cogs and open the bascules. These new engines were installed while the steam system was still in operation, so the Bridge could still open for passing river vessels.

In 1976, the work was completed, Tower Bridge became fully electro-hydraulic powered. It has remained the same ever since, and the old system was disconnected.