Science Puzzle
Why Does a Lift Have a Counterweight?
In most tall buildings the lift car hangs from steel cables that run up over a big grooved wheel at the top of the shaft and down the other side to a stack of heavy weights, the counterweight. The counterweight usually weighs about as much as the empty car plus half a full load of passengers.
Why hang all that extra weight in the shaft?
The Answer
The counterweight balances the car, so the motor only has to move the difference between the two sides, not the whole weight of the car and everyone in it.
Without a counterweight, the motor would have to haul the full weight of the car and its passengers up every time, then hold all of it back on the way down. With one, the lift works like a seesaw with people on both ends. When the car is about half full, the two sides weigh almost the same and the motor needs only a small effort to move them, rather like nudging a balanced seesaw.
That means a smaller motor, far less electricity and less wear on the brakes. Some modern lifts go further: when the heavier side is going down, the motor works as a generator and feeds electricity back into the building.
The counterweight has a second job too. Its weight presses the cables into the grooves of the wheel, so the wheel can grip them and drive them without slipping. As for safety, lifts have had automatic brakes since Elisha Otis demonstrated his safety catch in New York in 1854. Standing on a raised platform, he had the rope holding it cut, and the platform stayed put.
The principle: Counterbalancing. A counterweight on the other end of a lift's cables balances most of the car's weight, so the motor only has to move the difference, saving energy and effort.