The Technical Principle of Elevator II


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, such as center-opening, two-leaf, and single-slide. 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.

For freight elevators, vertically sliding, biparting doors are common. Such doors consist of an upper and lower leaf, mechanically linked so that the bottom half drops to floor level while the top half rises above the cab roof. A protective inner gate is often required.

In isolated locations, especially in private residences, a telephone to an outside exchange is often required by law. In many buildings, elevators have intercommunication systems in case of mechanical failures. Alarm buttons, emergency lighting, and emergency power are often provided.

Automatic loading and unloading devices have been incorporated into modern freight elevators. A call button activates the automatic pickup, when the elevator arrives, the load is pulled into the car, the car moves to the proper floor, and the load is discharged.

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