Elevators have been around for over 150 years.
In the 1800s, new iron and steel production processes revolutionized the world of construction. With sturdy metal beams as their building blocks, architects and engineers could erect monumental skyscrapers hundreds of feet in the air.
But these towers would have been basically unusable if it weren’t for another technological innovation that came along around the same time. Modern elevators are the crucial element that makes it practical to live and work dozens of stories above ground. High-rise cities like New York absolutely depend on elevators. Even in smaller multi-story buildings, elevators are essential for making offices and apartments accessible to handicapped people.
Most elevators work just like a pulley. A very strong metal rope is joined to the top of the elevator car and goes up through a “sheave” in the engine room above the elevator. The sheave is like a pulley wheel with grooves in it to hold the rope tightly. On the other side of the rope is a weight, which is about as heavy as the elevator car when it is half full. This balances the car, so that not too much energy is needed to move it.
Both the weight and the elevator car are held in place by guide rails at the sides of the elevator shaft (the tunnel the elevator is in). A motor can turn the wheel in either direction so that the elevator either goes up or down (with the weight doing the opposite). When you push the button inside the elevator, you activate the motor. When the motor stops, the grooves in the pulley wheel keep the rope in place so the elevator stops moving.
When you step into an elevator and close the door, you had passed through two doors and are now standing in a box (or the elevator car) inside a vertical passageway (called the lift shaft). One door is in the walls of the floor that you got off, and the other door is part of the car itself.
Inside the shaft are hoisting cables attached to the top of the car. The cables run over a sheave (pulley) connected to an electric motor at the top of the shaft. The other end of the cables is connected to a heavy steel weight called a counterweight. When the car goes up, the counterweight goes down; when the car goes down, the counterweight goes up.
How the counterweight reduces to a minimum the power needed to operate the elevator …
The weight of counterweight = Weight of the car + (about) ½ of its maximum passenger load
So when the elevator operates, it needs power only to lift the weight of the extra passengers in the car; the rest of the weight is balanced by the counterweight.