The Technical Principle of Elevator I


An early attempt to minimize the sacrifice of floor space in elevator installations in tall buildings was the basis of the idea of the double-deck elevator, first tried in 1932. Each elevator consisted of two cars, one mounted above the other and operating as a unit, serving two floors at each stop. The technique was being increasingly adopted at that time.

Modern elevators are made in a variety of types for many purposes. In addition to ordinary freight and passenger operations, they are also used in ships, dams, and such specialized structures as rocket launchers. Heavy-lift, rapid-descent elevators are employed in high-rise construction operations. Practically, all are electrically propelled, either by cables, sheave and counterweight, by a winding-drum mechanism (still used in many low-rise freight elevators), or by an electro-hydraulic combination. Multiple cables (three or more) increase both the traction surface with the sheave and the safety factor.

The drive motor usually operates on alternating current for slower speeds and direct current for higher speeds. With the direct-current motor, the speed is changed by varying the field strength of a direct-current generator, and by adjusting the direct connection of the armature of the generator with the armature of the drive motor. For high-speed elevators, a gearless arrangement is used, usually with the cables wrapped twice around the sheave.

The traction elevator may have an unlimited rise, rises exceeding 100 feet require compensating ropes, say, ropes from the bottom of the car to the bottom of the counterweight. As the car rises, the compensating rope weight is transferred to the car, and as it descends, more is transferred to the counterweight, keeping the load on the drive machine nearly constant.

Hydraulic cylinders and plungers are used for low-rise passenger elevators and for heavy duty freight elevators. The plunger pushes the platform from below by the action of pressurized oil in the cylinder. A high-speed electric pump develops the pressure needed to raise the elevator, and the car is lowered by the action of electrically operated valves which release the oil into a storage tank.

Specialized types of hydraulic cylinder and plunger arrangements, including horizontally placed elements, are used for unusual applications. For instance, the roped, or “geared”, type of hydraulic elevator common around 1900, with plunger and cylinder fitted with sheaves at each end, is employed on aircraft-carrier elevators to lift heavy loads short distances. As pressure is applied to the plunger, the distance between the sheaves increases, and the ropes wrapped around the sheaves pull up the elevator.

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