Elevator - A Vertical Transportation III


Hydraulic cylinders and plungers are used for low-rise passenger elevators and for heavy duty freight elevator. 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.

Elevators lifted by hoisting ropes are required to have platform “safeties,” devices designed to clamp onto the steel guide rails upon activation, quickly braking the elevator to a halt. The safety, usually mounted below the car platform, is actuated by a speed governor through a rope. The rope pulls the safety to the on position in the event of excessive downward motion of the car. The device first cuts off elevator power, and if excessive speed continues, it applies the safety brake.

Most modern elevators are automatic, using various control systems to operate elevators individually or in groups. The earliest automatic control system, single-automatic-push-button, gives a rider exclusive use of the car for a trip. It is used in small apartment buildings and for freight elevators.

Collective operation is popular for use with a single elevator in a building. The car answers all calls in one direction in sequence and then reverses and answers all calls in the opposite direction. It is used in larger apartments, hospitals, and small office buildings. A variation, called two-car or duplex collective, permits two cars to operate together and share calls between them.

Group-automatic operation controls two or more cars as a group, keeping them timed to operate within a specified operating interval. Group-automatic operation is used if traffic is heavy and two or more elevators are operating as in hospitals, department stores, and offices.

Separate outer doors and car doors are essential parts of modern elevator systems. The two usually employ the same type of operation. Doors are opened and closed by an electric motor on the car. Door speed in closing is regulated to avoid injury to persons caught in the closure. A sensor electrically reverses the door if it strikes an object in closing. Photoelectric controls and electronic proximity devices are also employed to control door reversal. Hoistway doors are designed so that they are always closed before the elevator can operate.

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