How Does an Elevator Work? A Counterweight and a Very Famous Brake

A conventional traction lift is a balanced system with a small number of components doing specific jobs:

• •The car, riding on guide rails that run the height of the shaft and keep it aligned
• •The counterweight, on the same rails system and the other end of the ropes, sized at the car weight plus about forty to fifty percent of the rated load so the motor is balanced for an average trip
• •Steel ropes, typically six or more for redundancy, passing over a grooved drive sheave at the top of the shaft; the car is held by friction between ropes and sheave rather than by winding
• •The motor and gearbox, or in modern installations a gearless permanent magnet machine compact enough to fit in the shaft, which is why new buildings need no machine room
• •The controller, which in a building with several lifts runs a dispatching algorithm deciding which car answers which call
• •The governor, a separate rotating device driven by its own rope, which senses the car's speed independently of everything else

Hoists existed in antiquity and nobody would ride one, because a broken rope meant death. Elisha Otis did not invent the elevator; he invented a safety brake in 1852 and, crucially, demonstrated it. At the New York Crystal Palace exposition in 1854 he had himself raised on an open platform and ordered the rope cut, and the platform dropped a few centimetres and stopped. His mechanism used a wagon spring held in tension by the rope; if the tension was lost the spring straightened and drove pawls into toothed guide rails. The modern equivalent works differently and to the same end: the governor, spinning with the car, detects an overspeed condition, grips its own rope, and that grip pulls a linkage that drives wedge-shaped safety gear against the guide rails, bringing the car to a controlled stop. It responds to speed rather than to rope failure, which covers more failure modes. There is also a buffer at the bottom of the shaft, and modern systems add an ascending car overspeed protection, since a counterweight falling is as dangerous as a car falling.

The public fear is of a cable snapping and the car plunging, and it is close to unfounded. Multiple ropes each capable of carrying the full load mean all must fail simultaneously, the ropes are inspected and replaced on a schedule, and the safety gear stops the car on the rails regardless of the ropes. The single well-documented case of a free fall in a passenger lift occurred in 1945, when a bomber flying in fog struck the Empire State Building, severing the ropes of a car in which an operator, Betty Lou Oliver, fell seventy-five floors; she survived, because severed rope coiled beneath the car and compressed air in the shaft acted as a cushion. The advice circulated about jumping at the moment of impact is useless, since a person cannot jump at anything close to the falling speed and would have to time it to a fraction of a second. The realistic hazards in a lift are doors and being stranded, and the correct behaviour when stuck is to wait for trained rescue rather than attempting to climb out, which is how most injuries occur.

In a tall building with multiple lifts, the interesting engineering is in dispatching rather than in mechanics. A naive system sends the nearest available car, which produces bunching, since a car that gets slightly ahead picks up fewer calls and gets further ahead. Traffic patterns differ sharply through the day, with a morning up-peak from the lobby, a lunchtime two-way pattern and an evening down-peak, and a system optimised for one handles the others badly. Solutions include zoning, where banks of lifts serve different ranges of floors so no car stops everywhere, double-deck cars serving two floors at once, sky lobbies in very tall buildings where passengers transfer between express and local systems, and destination dispatch, in which a passenger enters their floor at a lobby terminal before boarding and is assigned a specific car, which groups passengers going to the same place and substantially cuts stops per trip. The metric operators care about is not average wait but the proportion of waits exceeding a threshold, since perceived service is dominated by the worst experiences.

The building height at which walking up becomes intolerable is about six storeys, which is why pre-lift cities across the world converged on that height and why the top floors were the cheapest. The lift inverted the value of a building vertically, making the upper floors the most desirable, which is the origin of the penthouse. Combined with steel frame construction, which removed the requirement for load-bearing walls thick enough to hold up a tall structure, it made the skyscraper possible, and the two technologies arrived within a few decades of each other in the late nineteenth century. The remaining constraint is the rope itself: steel cable becomes too heavy to lift its own weight beyond about five hundred metres, which is why very tall buildings use transfer lobbies, and carbon fibre ropes introduced in the last decade roughly double the practical single run. Meanwhile the lift is statistically among the safest forms of transport, carrying a volume of passenger journeys that exceeds every other mode, in machines most passengers never think about.

A traction lift balances the car against a counterweight sized at the car's weight plus roughly forty percent of its load, so the motor moves only the difference and is held by friction on a grooved sheave. Otis sold the technology in 1854 by demonstrating a safety brake rather than a lift, and modern versions use a governor that detects overspeed and drives wedges against the guide rails. Free fall is essentially unknown, dispatching algorithms rather than mechanics determine waiting times, and the lift is what made floors above the sixth valuable.