Elevator Doors
The hardest working part of any elevator is the door system. Every time that elevator moves the doors are required to open and close. One of the major manufacturers states that eon average elevator doors open and close over 200,000 times per year. In most elevators that’s a lot of components moving and a lot of wear going on. I feel that doors are the most important part in the whole system for the simple reason that they are just about the only system the public interfaces with on a constant basis that can cause significant injuries. Some of you may want to argue about it but doors that are set to operate too quickly can hit passengers. If the safety devices are not operating properly they can catch a hand. Delaminating door skins can catch clothing or cause cuts. Believe me people can find some of the most unique ways to get injured on elevator doors that you can imagine.
There are many different types of elevator doors, some of which determine the type of elevator. Doors that open horizontally from the center are a part of freight elevators. Doors that slide vertically are associated with passenger or service elevators.
There are single panel doors, two panel three and four panel doors. These are known as single, two, three and four speed doors. They open in one direction and the panels fold under each other. The reason that we call them two, three and four speed doors is because each panel is moving at a different speed. Check it out next time you’re standing there watching a door open. The door that is furthest from the strike side will move slower than the one at the other end. This type of door is used to minimize the size of the hoistway while maximizing the width of the doorway. You see that when the doors of an elevator open they have to have somewhere to go and that’s normally into the hoistway.
There are also center opening doors and they can be either two single panels moving in opposite directions or two, three, and four panels on each side moving in opposite directions. Known a two (ot three or four) speed center opening doors.
In every passenger elevator you have a motorized car door that travels with, and is part of, the elevator car. At each landing or floor you have a set of hoistway doors. Hoistway doors are just dead panels. They have no capacity to open by themselves. They may be spring loaded to enhance closing and keep them closed but for the most part it’s just a sliding panel. Normally they are suspended on an overhead track. The doors have rubberized wheels attached to the top that roll back and forth on the track. Center opening doors are “related” by a cable. That means that because they are tied together with a cable whatever the position one door is in the other has to be in the same position as well. At the bottom of the door you have devices called gibs. Gibs are for lack of a better word, nubs that stick out from the bottom of the door. These” nubs” travel in a track in the bottom door sill. They clean debris out of the track by pushing it to holes in the sill, but their real purpose os to hold the bottom of the door in place and keep it from swinging in or out. The motorized car door engages the hoistway doors by means of a clutch assembly. The clutch sticks out just a bit from the car door and when the car descends the clutch engages two small rubber rollers that stick out from the hoistway door. To fit into the clutch as the car is coming down the rollers compress together which activates a mechanical apparatus that releases the lock on the hoistway doors. All hoistway doors have these locks as a safety measure to prevent the public from opening the doors and either getting hit by the elevator car or falling down the hoistway. Most elevators have a single clutch assembly that works the related doors but a few were built in the past that had clutches for each side of a center opening type.
Most passenger elevator doors are equipped with a safety device that will retract the door if it comes into contact with resistance from closing. That is if someone is standing in the doorway or if there is debris in the track that the gibs cannot push out. They operate on a mechanical basis and they have to be checked regularly by a technician to insure that they generate no more than thirty pounds of force before the door motion is reversed. Some do get out of adjustment and they can give you a nasty smack if you’re not careful. Many of these types are accompanied by a set of electronic eyes that are placed at 5 inches above the floor and twenty nine inches above the floor. All you have to do to retract the door is to break either beam of light. About twenty years ago the manufacturers improved on the beam idea by installing a device that projects many infrared beams from about 3 inches to 80 inches above the floor. These are non contact edges and are mostly what we see in elevators today. Although the doors still have to be set to retract at a maximum of thirty pounds in the event that the device fails. Elevator codes state that doors that are equipped with electronic eyes and infrared beams must include a feature called “nudging”. Quite a few electronic edges were installed when they first hit the market on elevators that didn’t have the nudging capability. Most of those have been weeded out through modernization and diligent state inspectors. The nudging feature is utilized in the event that the door is blocked by something or someone or mainly if the beam is obscured by smoke from a fire. After a set period of time an alarm sounds and the door will begin to close slowly. It still is set to no greater than thirty pounds of closing force in the event that someone may be lying in the doorway incapacitated.
Doors have so many moving and constantly wearing parts they account for at least 80% of the elevator failures today. Most of the time an elevator fails to operate due to a signal from a safety circuit. Think about it, for your elevator to operate the car door has to be closed completely and the contacts in the safety circuit providing a closed loop. Each set of hoistway doors also has a set of contacts that are tied to the safety circuit. If they don’t complete the loop due to something like an excess of pressure in the building, a worn closer that needs the spring tightened, corroded contacts in the circuit, or any other reason that everything is not closed up tight, the elevator will not run. We in the elevator business think that’s a good way to do things, we realize that the public is somewhat annoyed by this.
I’m out of time right now but I will be adding to this very soon. In the meantime don’t hesitate to leave a comment or send me a question at elevatorernie@hotmail.com.
Today’s quote comes from Will Rogers.
