How do elevators stay safe, and what did Otis invent?

Elisha Graves Otis did not invent the elevator. He invented the elevator brake, a device that stops a platform from falling if its hoisting rope breaks. Modern elevators descend from that idea but rely on a chain of electrical and mechanical safeguards, so that a fault at one point stops the car rather than letting it run on. Sources: invent.org, otis.com, explainthatstuff.com. [1] [2] [3]

By 1852 Otis was in Yonkers, New York, converting an old sawmill into a bedstead factory. A hoist was needed to lift heavy equipment to the upper floor, but workers were reluctant to use hoists because they were regarded as unsafe and sometimes crashed when the cable broke. Otis designed a safety device in response. [4] [1]

The device used a wagon spring attached to the platform and the lifting cable, with toothed guide rails on both sides of the shaft. While the cable was taut, the spring was held in place. If the cable broke, the tension was released and the spring threw its ends outward into the notches, stopping the platform. Archimedes is generally credited with an elevator-like device in the third century B.C., but elevators only became a safe means of moving people after this invention. One source dates Otis's work to the 1860s, while the other sources point to 1852. [5] [4] [2] [3]

Otis's company describes two sets of safety components. The electrical set includes the controller, which acts as the brains of the system, plus sensors that monitor car speed and position, door locks and access panels, and software that checks the redundant sensors. The mechanical set includes the machine, the machine brake, the overspeed governor, the car safeties and buffers at the bottom of the hoistway. [2]

The machine brake is spring-applied and lifted electrically, so it engages if power is lost. High-rise elevators hang on many cables, far more than needed to carry a full car. Each cable is made of many twisted strands, so a small failure does not cause immediate problems, and the other cables keep supporting the car if one fails. [2] [3]

Engineers talk about a safety chain: if any link is opened, the elevator stops until the issue is resolved. If a car exceeds its rated speed, the controller first changes the current to the machine to slow it, much like downshifting a manual car. If that is not enough, it cuts power and applies the machine brake. [2]

If the car still accelerates, the governor takes over. This speed-sensing device in the machine room has a rope attached to the car. When it spins too fast, flyweights swing outward and trip a mechanism that grabs the rope. The stopped rope pulls on the safeties, which clamp onto the rails and are the modern version of Otis's brake. Buffers at the bottom of the hoistway then absorb energy if the car still reaches the bottom, acting like large shock absorbers. [2]

A November 2018 incident shows the redundancy in practice. Only one of seven hoist ropes broke, and the other six allowed a slow, controlled descent from the 20th floor to the 11th, where passengers were rescued. An inspector's report said the car was never out of control or unsafe. [3]