Elevator Car Leveling

Elevator Leveling

Elevator leveling really doesn’t sound like much but it can be a huge issue for you the owner/property manager. A passenger who trips due to an offset elevator can be injured and sue the owner, the service company and just about anyone else in the neighborhood. Many elevator service companies negotiate their way out of the lawsuits with settlements that keep the costs lower than if they went to court.

Through the years many different schemes have been used to get an elevator car to stop evenly with the floor that it’s on.  Prior to 1950 we had elevator operators.  They actually “drove” the elevators. They had a handle to accelerate, slow down, and stop.  When you entered an elevator it was their responsibility to keep you away from the door, and then when the door was closed, release the brake, start the elevator in either the up or down direction, take you to your desired floor, slow it down, and finally bring it to a stop level with the floor.  Although it must have been a boring job it did take some skill to keep from undershooting and overshooting the floors. Of course if it wasn’t perfectly level the operator would caution you to watch your step when leaving the elevator.  All of that changed with the introduction of automatic service in the 40’s.

To begin the understanding of elevator leveling you have to know that each elevator is set up with a “profile” of how it’s supposed to run. When you push the button the door closes and the elevator is ready to start in either the up or down direction. The car starts to accelerate at a slow speed to avoid jolting the passengers. Think of it as a car taking off from a stoplight. If the driver floors it immediately you get pinned to the seat. Instead the driver will gradually pick up speed until he can shift to the gear that will take the vehicle to top speed. Elevators work the same way. The distance covered between the start and the shift to full speed is called the transition zone. They also employ transition zones in the slow down phase too, as sudden stops aren’t welcomed by most passengers.

Many elevators employ a system of mechanical switches in the shaft used to slow down then stop the elevator. The positions of the switches define the transition zones.  For traction elevators these switches, once set, do not have to be repositioned very often. They can lead to a mis-leveling condition if the car is heavily loaded or for some reason starts to run faster or slower.  If the switches are placed too close together the car will overrun the floor. When they are placed too far apart it increases the floor to floor time causing increased waiting time for the elevators.  Elevators use the same switches in either the up or down direction. Most elevators do not travel at the same speed in the up direction as they do in the down.  A switch will do double duty signaling for a slowdown as well as signaling to speed up.  Stopping the elevator is done by a third switch that tells the elevator that if it cuts the power right now, the car will stop right at the floor.  For those of you who follow baseball think of the stop switch signal as the batter suddenly deciding against hitting the ball in the middle of his swing. Sometimes he can stop it in time and sometimes not. That’s when the umpire calls a strike. If the elevator is traveling a bit too fast or slow the car will miss the floor. One quarter of an inch is acceptable, however that can still cause a trip and a nasty fall.

Some newer elevators use magnets rather than switches. When a pickup on the car passes a magnet placed in the shaft it sends a signal to the controller much the same as the switches mentioned above. In addition to magnets and switches some traction elevators use encoders to pinpoint the position of the car. These encoders are used to verify the position of the car in the shaft and they work in conjunction with switches, magnets or electric eyes to confirm the location of the elevator. Electric eyes are used on some models.  A metal strip with holes in it is hung the length of the shaft and the electric eye sender is placed on one side while the receiver is placed on the other. By this method the elevator can “count” the number of holes that it has passed in any one direction and maintain it’s location within ½ inch.

Additionally some older traction elevators use a cable that travels the length of the shaft with the car. It is tied through a system of gears to a selector. This selector turns at a reduced ratio like a screw. When the elevator arrives at a floor the technician has set switches on the screw to make contact to slow then stop the elevator.  These require constant adjustment due to cable stretching, and things working loose.

In the past control over the stopping of the motor determined much longer transition zones. With the introduction of computerized controls control is much tighter. This reduces the transition zone by a significant amount and results in a much faster floor to floor time for traction elevators.

Hydraulic elevators mainly use the system of switches. They experience different leveling problems mostly due to changes in the viscosity of the oil. If the hydraulic fluid is too cold or too hot it can make a huge difference as to where the elevator will stop. For instance if the serviceman sets up an elevator that is exposed to the outdoors on a frosty day it may not stop level to the floor on a hot summer day if the oil temperature has exceeded the parameters that define a certain viscosity.  Think of viscosity as thickness of the oil. When it’s cold it’s thick and much more difficult to pump. When it’s hot it’s thin and easier pumping translates to faster speeds.  A good example is driving your car, if you cut your ignition at 60 and roll to a stop you will go further than if you cut it at 30. The solution for a hydraulic elevator is to keep the oil within a certain temperature range either by re-circulating (more on this later), or with a tank heater.

This posting is not going to go into every way to stop an elevator, just some of the major methods in use today.  Suffice to say that stopping in the right place is a complex and important issue that you should be aware of and make sure that your serviceman is paying sufficient attention to.

I would like to ask you again if you have any questions, or comments. I have been thinking about possibly working on some software that can write your maintenance plan and tailor it specifically to your equipment and conditions. Please let me know if this is something you may be interested in.  Also if there are any subjects you would like me to tackle next don’t hesitate to comment or email. Otherwise I plan to write a bit about elevator controllers in the next posting.  For questions or comments please contact me at elevatorernie@hotmail.com.

Thank you,

Ernie

Quote of the Day

“An ignorant person is one who doesn't know what you have just found out.”   - Will Rogers