Elevator - The Designs and Materials I

Date:2016-09-19

Elevator themselves are simple devices, and the basic lifting systems have not changed much in over 50 years. The control systems, however, have changed substantially to improve safety and speed of operation. Elevators are designed for a specific building, taking into account such factors as the height of the building, the number of people traveling to each floor, and the expected periods of high usage.

Most elevators use counterweights which equal the weight of the elevator plus 40% of its maximum rated load. This counter-weight reduces the weight the motor must lift and ensures that the elevator cannot fall out of control while the cable is intact.

In a lifting drum installation, a hoist cable runs down from a drive drum attached to the hoist motor, around a large pulley on the top of the elevator, up to a second pulley hanging from the roof of the elevator shaft, and down again to the counterweight. In a traction drum installation, the cable runs from the elevator, up and once around a drive drum attached to the hoist motor, then back to the counterweight.

The elevator, called the car, and the counterweight each run in their own sets of guide rails. A second governor cable runs from the car up to a governor pulley, then down to a tension pulley at the bottom of the elevator shaft, and up to the car again. This cable rotates the governor pulley at a speed directly proportional to the speed of the car. In the event of excessive car speed, the governor uses another cable to activate the emergency brake jaws which grip the guide rails and slow the car to a stop.

A ramped bar on the side of the elevator shaft activates a series of switches on the outside of the car to slow and stop the car at the proper floor. As the car approaches the desired floor, the ramp activates the slow-down switch, which signals the hoist motor to reduce speed. When the car is aligned with the outer door opening, the ramp activates a limit switch to stop the car. If the door interlock switches also sense that the car is in the proper location, the electric door opening motor is activated to open both the inner car door and the outer floor door.

Modern commercial buildings commonly have multiple elevators with a unified control system. The object of the control system is to minimize the average time any passenger spends from the time the elevator call button is pushed to the arrival of the first available elevator. Different systems use different levels of sophistication. The simplest systems use a single up and down button on each floor regardless of the number of elevators. When a passenger calls for an elevator, the controller sends the nearest elevator that is traveling in the desired direction. The approach of an elevator car is signaled by an illuminated arrow above the elevators doors pointing up or down.

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