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Elevator Safety Features

elevator shaftElevators are built with several redundant safety features to protect riders.

Cable-driven elevators have multiple (four to eight) ropes to support the weight of the car and passengers. The cables consist of several lengths of steel material wound around each other. One cable is capable of supporting the weight of the elevator car and the counterweight. The extra ropes ensure that even if one cable snaps, the elevator will not free fall.

Even if all the ropes were to snap, or if the sheave system they were wound around released them, the elevator car would probably not free fall to the bottom of the shaft. Cable-driven elevators have safeties, a built-in braking system that grabs onto the rail if the elevator moves through the shaft too quickly. Safeties are activated by a governor, which is built around a sheave at the top of the elevator shaft. The governor rope loops around the governor sheave and another sheave at the bottom of the shaft. The rope is connected to the elevator car and moves when the car ascends or descends. When the car speeds up, so does the governor.

The sheave has two hooked flyweights, or weighted metal arms, that are held in place by a high-tension spring. If the elevator car falls too fast, the centrifugal force pushes the flyweights out and forces them to catch on ratchets, which stops the governor. A movable actuator arm attached to a lever linkage connects the governor ropes to the elevator car. If the governor sheave locks itself, the governor ropes jerk the actuator arm up, which moves the lever linkage, which then applies the brakes.

In other designs, a wedge-shaped safety sits in a stationary wedge guide. When the wedge moves up, the slanted surface of the guide pushes it into the guide rail, which applies the brakes.

Elevators have electromagnetic brakes that engage when the car stops and keep the brakes in the open position. If the elevator loses power, the brakes will clamp shut. Elevators also have automatic braking systems at both ends of the shaft that will engage if the car moves too far in either direction.

If all of these safety features fail and the elevator falls down the shaft, a heavy-duty shock absorber system, usually a piston mounted on an oil-filled cylinder, will act like a cushion to soften the landing.

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