Elevator plunges are rare because brakes and cables provide fail-safe protections


It takes a freak event to sever all of the cables supporting an elevator. (ISTOCKPHOTO)
June 10, 2013

How many times a day do you step into an elevator? Many people live and work 10, 20 or even more stories above the ground. Yet rarely do you think about complicated electromechanical systems that glides you up and down, lest your mind wander to thoughts of a Tower of Terror-like plunge into the subbasement.

If you understood exactly what’s stopping you from plunging 500 feet into the subbasement, would you be more or less comfortable riding an elevator? There’s only one way to nd out.

Let’s begin with the cables. Most elevators feature between two and eight woven steel cables. Elevator technicians refer to them as “ropes,” a reference to their 19th-century hemp predecessors. The number of ropes in a given elevator depends on something called a “factor of safety.” If the factor of safety, set by building codes, is 12 for a particular building, that means the combined strength of the ropes must be adequate to hold 12 times the mass of a fully loaded car. In effect, each rope can hold more than the weight of the car.

Individual cables occasionally fail, but it takes a freak event to sever all of them. In 1945, a B-25 bomber crashed into the Empire State Building, slicing through all of the cables on an elevator. The lone passenger survived the fall from the 79th floor because the cables beneath the cab slowed her descent and cushioned her landing. The planes that crashed into the World Trade Center on September 11, 2001, also sliced the elevator cables, and some of the victims plunged to their deaths.

Elevator engineers worry about more than just cable failures: The electronics, the pulley systems and other features must be working to ensure safe travel.

“Before each run, the elevator system checks the ‘safety chain,’ ” notes Daryl Marvin, director of innovation at Otis Elevator, the world’s largest elevator maker. (He was referring not to a physical chain but to a series of checks the elevator performs automatically.) “If anything goes wrong — the door is open, the elevator detects an overspeed or someone presses an emergency stop button — the system automatically cuts power to the motor and applies the brake.”

Elevators have two or three types of brakes. If there’s an error in the safety chain, a clamp closes on the pulley above the car, preventing the elevator from moving. Unlike an automobile brake, which has to be depressed to engage, the elevator brake is clamped down unless power is supplied to release it. That means that any loss of power, either due to a system error or an electrical grid failure, will set off the motor brake.

The safety check and the motor brake have failed on occasion, but negligence is the usual cause of accidents. In 2011, for example, an elevator in a Manhattan office building surged upward with the door still open, killing a 41-year-old advertising executive. An investigation showed that maintenance workers who disabled the safety chain during repairs forgot to reset the system.

Elevators also have a safety brake that is attached to the underside of the car. This is the innovation that made the passenger elevator possible when it was unveiled at the 1853-54 World’s Fair in New York.

“Before Elijah Otis invented the safety brake, elevators were only used for freight,” Marvin says. “Ropes broke sometimes, and without any backup it would be crazy for a passenger to take that chance.”

Here’s how the safety brake works. If the electronics detect that the car is speeding downward, it jams a metal brake from underneath the car into a channel in the guide rails, the metal rods along which the elevator travels. Friction builds between the wedge and the rail, which brings the car to a stop at a comfortable rate.

There is one more fail-safe. On the opposite end of the cables that attach to the elevator car, there is a set of counterweights. Those weights weigh slightly more than an empty car and slightly less than a fully loaded car. If every other safety system failed and you were the only person in the car, these weights would make the elevator ascend rather than descend. It would happen slowly at first, gaining speed as the ascent continued. A fully loaded car would experience a slowly accelerating descent.

In either case, when the counterweights reached the top or bottom of the shaft, they would meet a cushion that would bring the elevator car to an abrupt but hopefully survivable stop. “It wouldn’t be pleasant, but you have a very, very good chance of being fine,” Marvin predicts.

Many of the safety systems on modern elevators are fundamentally similar to the ones used 100 years ago, with refinements to account for the increasing speed and weight of today’s cars. The fastest elevators now travel around 30 mph. (The descent is slower than the ascent, because rapidly increasing air pressure can cause discomfort in passengers’ ears.)

Many of the updates have to do with materials. Steel would buckle under the heat created by a heavy elevator car falling fast down a skyscraper shaft. Marvin wouldn’t tell me what Otis uses instead, but he noted that the material is the same designed to withstand the heat of a jet engine. Many of the new models are tested in one of Otis’s test towers, which are exactly what they sound like: buildings made up almost entirely of elevators. At least the engineers never have to wait in the lobby.

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