How a plunging lift halted

The world’s tallest office building, the 1454ft (443m) Sears Tower in Chicago, has 103 lifts to whisk passengers between its 110 storeys at speeds up to 1800ft (550m) a minute.

But what would happen if the cable broke while a lift was high up such a tall building? A body falling from the top storey of Sears Tower would, in theory, smash into the ground at around 200mph (320km/h). To avoid disasters, lifts are designed with safety mechanisms.

The development of the modern passenger lift has its origins in 1854, when the American engineer Elisha Graves Otis introduced the first fail-safe mechanism for freight hoists at the Crystal Palace Exposition in New York. Previously, hoists had been notoriously unsafe. Ropes used to hoist goods on platforms frequently broke – sometimes killing people.

Something of a showman, Otis demonstrated his safety hoist in a dramatic way. The hoist was hauled up about 25-30ft (7.5-9m) with him standing on its platform, along with boxes, barrels and other freight. Then he ordered the rope to be cut. On earlier hoists it would have been disastrous. But Otis’s fail-safe mechanism worked – and the hoist stopped dead after the rope was cut.

The secret of Otis’s success was a bow-shaped spring attached to the top of the hoist platform. As the platform was hauled upwards, the spring arched and its ends did not make contact with notched guide rails on each side. But when the hoist rope was cut, the spring flexed back and its ends jammed in the guide rails, preventing the platform from plummeting down.

Otis installed the world’s first passenger lift in New York City at the five-storey Broadway china store of V. Haughwout & Co, in 1857. The invention of the safety lift was a key factor in the development of the skyscraper. Previously buildings had been limited to six storeys, because people baulked at climbing too many stairs.

The passenger lift freed architects from height restrictions. And the development of iron-frame building techniques in the 1880s provided the means for high-rise construction.

A modern passenger lift is not fundamentally different from Otis’s original. It is a boxed-in platform, hoisted by steel cables between two guide rails, and has a safety mechanism that jams into the guide rails if the cables break.

The cables lead from the lift car up and over a pulley device called a sheave, at the top of the lift shaft. The pulley is driven by an electric motor. The cables lead down to a counterweight, also on guide rails.

Overspeed governor

A key component in lift safety is an overspeed governor. A cable from the governor runs up and down the lift shaft and is attached to the safety gear mounted beneath the floor of the lift car.

The overspeed governor relies on centrifugal force, which causes a system of weights to swing outwards. Beyond a preset speed, their weight activates a safety switch, cutting off power to the drive motor. The pulley is then automatically braked, and the lift stops without having to activate the safety device.

However, if the car continues to accelerate, the overspeed governor will clutch the governor rope with sufficient force to trip the safety gear.

Rapid ascent

The 110-storey Sears Tower in Chicago has high-speed lifts which travel at up to 20mph (32km/h). The lifts are fitted with safety devices in case the cables break.