Traction Elevators

Traction Elevator Basics

Traction elevators are suspended from cables. The reason that they’re called traction elevators is because the traction between the cable and the sheave allows the car to move up and down with the motion created by the motor. The cables are called ropes and for a very good reason. Originally some elevators were installed with manila ropes. There are a very few still around today. Today’s steel cables actually have rope material inside of them to carry lubricants to the cable to minimize wear.  The wear comes in when the rope moves up and down over the sheave (pulley). It twists as it changes direction and the strands in the cable rub against one another.  Almost all traction elevators are elevators are suspended by more than one rope for safety purposes. Each individual rope is capable of holding a fully loaded car. If all of those fail the governor rope is employed to suspend the car. On average each elevator has three to five ropes attached.  One end is attached to the cab while the other end is attached to a set of counterweights to balance the load. Note: Drum type machines do not have counterweights and the cable is wound onto a drum, sort of like your fishing reel. There are a few of these still in use but they are no longer manufactured in the US. Traction machines are either installed on the roof or in the basement.  Most of them have 1 to 1 roping configurations meaning that with the cable draped over the pulley the counterweights are on one side and the car is on the other.  For every turn of the drive sheave the elevator will travel the circumference of the sheave.  In a 2 to 1 configuration the ropes go under the car and the elevator is suspended on a loop in the ropes. Thus for every two turns of the drive sheave the elevator only travels the distance of one turn.  This reduces loads on the motor and enables smaller faster motors to be used. 

Traction elevators are either geared or gearless, that is they either work on a system of gears to provide power to the sheaves or in the case of gearless work directly off the motor.  Geared elevators are limited in hoistway speeds therefore are mostly used in low to mid rise buildings. The machine in a geared elevator consists of a worm and ring gear that reduces the motor speed to convert it to the power required to lift the car.  In this configuration smaller motors are used to reduce cost and power requirements. High rise buildings require faster speeds.  The sheave is actually part of the motor with no reduction gears. The motors are larger to provide the horsepower needed.  

Earlier models were built with almost all direct current motors.  The direct current allows greater control of the motor to be able to slow down gradually when coming in to a floor so as not to bypass it. Inversely they are also able to start slow and ramp up speed to avoid starting with a jolt.  Two speed AC motors were used in some installations. They worked but not really all that well. Any of you that have been to Europe and ridden some of the older elevators may have experienced a ride in an AC unit.  When the elevator stops you feel like you have just landed on a bowl of jello with a boinnnng type of effect. It’s only due to recent technology that we have been able to control AC to such a degree that the elevator will stop and start accurately every time.

Main components of traction elevators consist of:

1.       A main control device that turns the power on and off to the motor.

2.       A motor (of course)

3.       Cables that hook up to a counterweight system

4.       A passenger car

5.       Sensing devices that signal the controller

6.       A device that controls power to an AC motor or a device that generates DC power to a DC motor

Many people think that Elija Otis invented the elevator. All that Otis did was develop a safety brake that enabled humans to ride in elevators safely.

In the next posting we can go into some of the common items found on both major types of elevators. As always, please don’t hesitate to contact me with any questions or comments that you may have. I will make your questions the subject of future publications to make sure that everyone is fully informed.  Send your questions/comments to elevatorernie@hotmail.com. I am looking forward to hearing from you.

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