Elevator and Escalator Consulting Engineers
In the early days of elevators, doors were manually operated. Even today there are some residence elevators or handicap lifts which have manually operated doors.
However, with the advent of the automatic elevator it was obviously desirable that the doors be motorized.
The door operator consists of an electric motor generally with some type of speed reduction system (either gears or belts), mechanical linkages to the car doors and a motor speed control system.
It was apparent to the elevator designers that a door operator for each entrance was an expensive proposition and for that reason the "master door operator" was developed.
The master door operator uses one door operator on the car to open and close the car doors. The car doors in turn are mechanically linked to the hall doors so that when the elevator is stopped at a particular landing the hall doors for that landing open and close in synchronism with the car doors.
Although a bit difficult to describe, the concept is simple. A typical design uses a "skate" or "vane" mounted on the car cab door. This is a piece of steel about 12 mm thick (1/2"). On the hoistway door two rollers are mounted, located so that the car door vane runs between them as the elevator goes up and down the hoistway. The typical clearance between each roller and the car door vane is 6 mm (1/4"). When the elevator stops at a floor and the car door opens, the car door vane pushes against one of the rollers, tripping the hoistway door lock open and moving the hoistway door together with the car door.
Other designs (e.g. GAL) use "clutches" mounted on the car cab doors arranged to engage rollers on the hoistway doors.
As can be seen, there are some rather small clearances given that the elevator is running past the doors at speeds up to 9 meters per second (1800 fpm). If something goes wrong with the guiding devices that restrict the lateral movement of the car cab the vane will sometimes touch the hoistway door rollers when running past the doors at full speed. This can cause the hoistway door lock to open, interrupting the safety circuit, and the elevator will lose power for an instant. If you are riding the car you might notice a sharp blip in the car motion and perhaps you may hear the noise of the vane clipping the roller.
Common faults are: a door failing to close; a door recycling; a door hitting people as they go through the entrance; the electrical contact used to indicate the door is closed, failing to complete the electrical safety circuit even though the door appears to be closed.
Generally the door fails to close because the door detector (the device that monitors the opening to ensure that the doors do not close when something or someone is in the entrance) fails. This can commonly occur with dust on the detector surface (which prevents the infra-red light from getting through). If there is any construction work going on in the building then a more frequent cleaning of the detectors is required. Although the maintenance mechanic may do this, it is a good idea for the building staff to clean the detectors when cleaning the car cabs. They should receive instructions on this from the elevator maintenance contractor.
Doors often recycle because the lock does not make up and a control circuit designed just for that purpose tells the doors to "try again" by opening and closing. Sometimes, as well, the detectors "false fire" when the doors are almost closed. Both of these are maintenance problems which should occur only rarely if proper routine maintenance is being performed. Doors hitting people can result from a faulty door detector device or from incorrect door operator speed settings. This is a serious fault since it may result in injury and litigation. For this reason immediate action is required; it is a good idea to shut the elevator down until the fault is corrected.
Electrical contacts on the door locks failing to make up can be frustrating for everyone. Frequently this is an intermittent fault and as a result somewhat difficult to pinpoint. However, good routine maintenance on the hoistway doors will minimize this type of failure to the point that it virtually never happens.
Close to half of the trouble calls on elevators are related to the door operators and door equipment. Concentrating on first class maintenance in this area will pay dividends in passenger satisfaction.