Like the automobile, the residential elevator is a mechanical marvel—and a fairly recent invention. If you’re a board member, a manager or just a unit owner of an urban or suburban co-op or condo development, chances are you probably use an elevator every day. We’ve all seen those elevator inspection reports but chances are that we don’t think about the inner workings of elevators very much. And it seems like the only times that elevators make the news is when something goes wrong, like the time a Chinese-food deliveryman was stuck for three days inside an elevator in a Bronx high-rise.
There have been elevator-like hoist devices throughout history, but in 1853, American inventor Elisha Otis invented a freight elevator equipped with a safety device to prevent the elevator from falling in case a cable broke. This led to widespread use of the equipment in the commercial and residential sectors of society.
How They Work
Today, there are basically two types of elevators in use—hydraulic and “rope-driven.” If you’re visualizing a rope similar to the one you used to climb in gym class, however, you’re way behind the times—today’s rope driven elevators contain six to eight lengths of wire or steel cable. These cables are attached to the top of the elevator cab and wound around a drive sheave in special grooves. The other ends of the cables are attached to a collection of metal weights equal to the weight of the car and a little less than half of its rated load. The counterweight slides up and down the shaftway on its own guide rails.
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. These motors are able to control speed, and allow for the elevator's smooth acceleration and deceleration. Signal switches also stop the cab at each floor level.
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 triggers the electric motor to gradually shut off the pump. To get the elevator to descend, the control system sends a signal to the valve operated electronically by a switch. When the valve is opened, the 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 down.