If the hydraulic elevator is the plow horse, always reliable over the long haul, moving at a steady jog, then the traction elevator is the racehorse. Like any thoroughbred, the traction elevator is designed for speed and distance. While hydraulic elevators are moved by pistons, traction elevators are moved by ropes and pulleys.
If you remember your 6th grade science class, you will understand how these racehorses run. The car is raised and lowered through the hoistway by ropes (really strong braided wire cables) that pass over a grooved wheel or sheave (a fancy name for a pulley) that is attached at the top of the hoistway. In its simplest design, one end of the rope is attached to the elevator car and the other is attached to a stack of counterweights. The weights help balance the load, making it easier for the motor to drive the sheave forward and backward, moving the passengers up and down to their chosen floors.
If you have the chance to see a traction elevator in action from on top of the car, you will notice there is more than one rope and often multiple sheaves. The reason for all of the ropes is for safety. If one breaks there are others to keep it from falling. Also, if you weren’t sleeping during that 6th grade science class, you will remember that the more pulleys or sheaves between the load and the effort, the easier it is for the motor to raise and lower the car. Multiple ropes make the system safer and multiple sheaves make it more efficient.
There are three distinct sub-groups of traction elevators: Geared Traction, Gearless Traction and Machine Roomless (MRL).
Geared Traction Elevators have a motor in a machine room above the hoistway, and a gearbox attached to the motor, which then turns the sheave. The gears in the box amplify the motor’s power, allowing for a smaller motor than a gearless machine. As we all know when we are waiting on an elevator, speed is important and geared traction elevators can travel up to 500 feet per minute (fpm) while hydraulics max out at 200 fpm. They can travel up to 250 feet, while hydraulics start getting much more expensive at about 60 feet.
If you think a geared traction elevator is quick and can reach new heights, its brother, Gearless Traction, tears up the track. These elevators have the sheave attached directly to the motor and can thus travel farther and faster than geared traction or hydraulic. This is the type of elevator you will find in high-rise buildings as they can travel up to 2,000 fpm and up to 2,000 feet total.
Whether geared or gearless, traction elevators need to have a machine room at the top of the shaft and their powerful motors can burn up to 10% of the total energy needs of a building. A more economical and ecological solution was needed and as a result the Machine Roomless Elevator or MRL was born.
MRLs are traction elevators and can look just like gearless traction elevators with their ropes and sheaves, until you get to the top of the hoistway. Up there you will find a much smaller motor and sheave located in the overhead space (above the travel distance of the car), off to the side over the counterweights. The placement of the motor is possible due to miniaturization and increased efficiency. The controller is located in a small room outside the hoistway, typically at the top stop. MRLs can run circles around hydraulic elevators, with speeds and travel limits similar to geared traction. MRLs are a popular choice because they are more energy efficient than overhead traction.
Whether your building should have a plow horse or a racehorse will be addressed in a later post. But before then, just in time for Halloween, we will introduce the Frankenstein’s Monster of elevators, the roped hydraulic, and explain how this unholy creature functions.