When an elevator lifts you almost instantaneously to the 60th floor, you feel you must be traveling at a tremendous speed.
Actually, the elevator rises 800 feet in 30 seconds, or only 18 miles an hour. Compared with the slow hydraulic elevators used in buildings of five stories and less, however, this speed is indeed fast.
Tall office and apartment buildings use electric elevators, and one of the fastest in the country is located in the John Hancock Building in Chicago. That elevator can travel 900 feet in 30 seconds.
A modern electric elevator is operated by a gearless traction machine, which is based on a system of hoists first developed by the Egyptians to build the Pyramids.
Six to eight steel cables, or hoisting ropes, are attached to the top of the elevator car. They extend to the top of the hoistway, where they are wrapped around a grooved wheel, or drive sheave, measuring 30 to 48 inches in diameter. The ends of the cables then drop back down the hoistway, holding a counterweight that slides up and down on its own guide rails.
The weight of the elevator car on one end of the ropes and the total mass of the counterweight on the other end press the cables down into the grooves of the drive sheave. A large, slow speed electric motor turns the drive sheave at a rate of 50 to 200 revolutions per minute, thereby moving the cables and lifting the elevator.
The electric hoisting motor does not have to lift the full weight of the elevator car and its passengers, for the weight of the car and about half its passenger load is balanced by the counterweight, which slides down as the car goes up.
Any elevator that travels more than 250 feet in 30 seconds requires additional traction, which is achieved by wrapping the hoisting ropes around a secondary gear like sheave located just below the main drive sheave.
