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How Do Cable Elevators Work?

cable elevator photoRoped or cable-driven elevators are the most common design type. They use traction steel ropes and a counter-weight to raise and lower the car.

The cables are attached to the elevator car and looped around a sheave, a pulley with grooves around the circumference to grip the ropes. When the sheave is rotated, the ropes move, which allows the elevator car to ascend or descend. The sheave is connected to a motor that turns the sheave in one direction to raise the elevator and in the opposite direction to lower it.

In a gearless elevator, the motor rotates the sheave directly. In a geared elevator, however, the motor turns a gear, which in turn rotates the sheave. The sheave, the motor, and the control system are generally contained in a machine room at the top of the elevator shaft.

The cables are attached to a counter-weight that hangs on the opposite side of the sheave. The counter-weight usually weighs 40 percent of the weight capacity of the elevator car. If the elevator is 40 percent full, the weight of the car and the counter-weight are balanced. This arrangement conserves energy by keeping the potential energy of the system near constant. If the weight of the car and the counterweight are balanced, only a small amount of energy is required to move the car in either direction. The motor only has to overcome the friction of the ropes on the sheave. The system works on the same principle as a see-saw.

The elevator car and counter-weight ride on guide rails along the sides of the shaft that prevent the car and counterweight from swaying and help to stop the car in the event of an emergency.

Cable-driven elevators are more versatile and efficient than hydraulic elevators and generally contain more safety features.


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