Flight or Fancy The Workings and History of the Elevator

In the future, we might have molecular transporters or some other newfangled invention defying the laws of gravity, but for some 150 years, Elisha Graves Otis' method of transport - the elevator - has stood the test of time.

Since the beginning, mankind has always had the need to move animals, objects or materials from one place to another. Historians believe that it was probably the Egyptians who first developed an intricate hoist system to build the pyramids, as many of the building blocks in the Great Pyramid of Cheops at Giza, Egypt, weighed well over 200,000 pounds. Some accounts cite that 4,000 stonemasons and 100,000 laborers were needed just to lift the huge stones into place. These hoisting inventions provided the genesis of what is today the modern elevator.

Behind the Machinery

There are two basic types of elevators in use today - hydraulic and rope-driven. A rope-driven elevator contains six to eight lengths of wire or steel cable - the hoisting ropes - which are attached to the top of the cab and wound around a drive sheave in special grooves. The other end of the cables are attached to a counterweight - a collection of metal weights equal to the weight of the car and about 40 to 45 percentage of its rated load - that slides up and down the shaftway on its own guide rails. So the weight of the elevator and the total mass of the counterweight acts in balance - sort of like a see-saw - to press the cables down on the grooves of the drive sheave. In a gearless traction elevator, the motor rotates the sheaves directly; in geared elevators, the motor turns a gear train that rotates the sheaves.

Chains or cables loop through the bottom of the counterweight to the underside of the car to help maintain balance by offsetting the weight of the suspension ropes. Guide rails that run the length of the shaft keep the car and counterweight from swaying or twisting during travel. Rollers are attached to the car and the counterweight to provide a smooth ride along the guide rails. An electric motor then turns the sheave, producing enough revolutions per minute to make the car slide up or down. These motors are able to precisely control speed, and allow for the elevator's smooth acceleration and deceleration. Intricate signal switches also stop the cab at each floor level. Roped elevators are much more versatile and efficient than hydraulic models, and typically have more safety features.

In a hydraulic elevator, the car is lifted by a hydraulic-fluid driven piston mounted inside a cylinder. The cylinder - containing oil or a similar substance - is connected to a pumping system. The pump forces fluid into the tank leading to the cylinder; when enough fluid is collected, the piston is pushed upward, lifting the elevator car on its journey. When the car is signaled that it is approaching the correct floor, the control system will trigger the electric motor to gradually shut off the pump. To get the elevator to descend, the control system will send a signal to the valve operated electronically by a solenoid switch. When the valve is opened, the acollected fluid flows out into a central reservoir, and the weight of the car and its cargo pushes down on the piston, driving more fluid out and causing the cab to move downward. The control system will signal the valve again for it to stop at the correct floor.

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