FSP0024 – Elevators Part 2 – Facility Science Podcast #24

Safety mechanisms related to the doors prevent opening of the doors while the car is not properly leveled at a landing or prevent any movement of the car while the doors are open.

Safety mechanisms related to the car falling work to prevent the car from crashing into the pit at a speed high enough to kill the passengers. I read a good article about this from Otis elevators. It looks like it might have been in a magazine at one point, but I read it on their web site. The article is titled “Safety Chain” with the subtitle “A look at safety systems in a modern high-rise elevator in light of the 2018 Chicago incident.” I’ll reference that article a little bit here. The “2018 Chicago incident” referred to there in the subtitle is, reading from the article here, “In late 2018, an express elevator in a Chicago skyscraper was traveling down from the 95th floor to the lobby when it came to an unexpected stop away from a landing. Investigators found that one of the elevator’s seven suspension cables had broken, likely as the elevator was passing the 20th floor. The car slowed to a safe, controlled stop between the 12th and 11th floors. The six passengers inside were unhurt. The incident immediately generated sensational media headlines, variously claiming that the elevator at 875 North Michigan Avenue (formerly the John Hancock Center) had plummeted, plunged or fallen 84 floors. It hadn’t.” (And unfortunately they left out the Oxford comma in that last sentence). The article then goes on to explain that the elevator didn’t actually plummet, or plunge, or fall 84 floors (from the 95th to the 11th), but instead came to a safe, controlled stop in 8.5 floors (from the 20th to the 11th) exactly as designed because of the safety features designed into the elevator…which, of course is exactly what an article written by an elevator manufacturer would say. It is a good article and very concisely explains the mechanisms involved. Link to the article: https://files.otis.com/otis/en/us/assets/documents/JUNEMAG%2073.pdf

Controller – first we have the controller. The controller can monitor many things, but in the most basic case it might monitor the speed of the elevator. It can actuate the speed of the traction machine motor to adjust the speed (in this context to slow it down if the car is moving too fast). The controller could also activate some of the other safety mechanisms if it can’t get control of the car’s speed just by adjusting the motor.

Traction Machine – The traction machine itself acts as a safety mechanism in that it can be slowed down in order to slow the car down.

Machine brakes – there is also a brake built into the traction machine that can stop the sheave from turning. This is similar to the disc or drum brakes in a car. The brakes can be applied by the controller if necessary. Also, the brakes are held open electrically, so if power to the elevator fails, the brakes will be applied automatically stop the car safely.

Governor – next we have a device called a governor. The governor is a device consisting of a sheave and a cable (called the governor rope), (and it’s actually 2 sheaves, one at the bottom of the hoist way and one at the top). But the actual governor devices is at the top. These sheaves and the governor rope are separate from the traction machine and the hoisting cables. The governor rope is also attached to the car. The governor rope goes around and around as the car moves up and down. The governor’s job is to detect when the car is moving too fast. It does so by spinning as the car is moving. The faster the car is moving, the faster the governor rope will move over the governor sheave. When the governor gets to a certain speed, centrifugal force will cause some type of weighted or spring loaded mechanism to spring outward activating the governor mechanism. The governor will engage the machine brake (so now we have 3 different ways the brake can be activated) and also the car safeties (which I’ll take about in a minute). The safeties are engaged mechanically, so no electric power is needed for this. When the governor overspeed mechanism trips, the governor rope is stopped. Since the car is presumable still moving at this point, governor will pull on the the safety level on the car causing the safeties to deploy.

Safeties – The safeties are a device mounted on the car and riding on the guide rails. When the safeties are engaged, they grip onto the rails and bring the car to a stop relatively quickly. And in case I didn’t make it clear enough, the operation of the governor and deployment of the safeties is an entirely mechanical process. This doesn’t involve the elevator controller or even require any power.

Finally, at the very bottom of the hoistway, in the pit, we have the buffer (I talked about this a little in part 1). The buffer is there as a last resort if all of the other safety mechanisms fail to stop the car before it reaches the bottom. The buffer is some type of hydraulic spring designed to absorb or otherwise dissipate some of the energy of a falling elevator car.

In addition to these mechanisms, we can also count the hoisting ropes and maybe even the counterweight as safety mechanisms. Elevators ropes are highly redundant so the car won’t fall even if most of the ropes break, and in many scenarios, the balance between the car and the counterweight will slow any possible acceleration which means the car will take longer to reach dangerous speeds.

Hydraulic vs traction – Other than the buffer, the safety mechanisms I described here don’t apply to hydraulic elevators (which is most elevators. I read somewhere that something like 75% of elevators in the US are hydraulic elevators, which makes sense since most building with elevators are less than 8 floors). The reason hydraulic elevators don’t have (and generally aren’t required to have) these fall protecting safety features is partially because they don’t go as high or as fast as traction elevators. Less distance to fall and less speed means less energy in any collision. Another big reason is that a hydraulic elevator doesn’t hang from cables, instead it is supported from underneath by a hydraulic piston making actual free-fall nearly impossible (nearly impossible, but not impossible). Also, a hydraulic elevator can’t fall upward, unlike a traction elevator where the car can “fall” upward if is lighter than the counterweight. Most of the time, a failure in a hydraulic system will be caused by leak of hydraulic fluid with the result being that the elevator car sinks slowly. This can be a safety issue because it can cause mis-leveling of the meaning that after the car stops at a landing and opens the doors, the car will continue to sink a little bit creating a trip hazard and, much more rarely, a crushing hazard. It would take a catastrophic failure of the hydraulic piston to cause a hydraulic elevator to fall into the pit with enough speed to injure people. Most commonly this happens when the bottom cap of the cylinder breaks out causing the hydraulic fluid to leave the cylinder fast enough for the car to gain some speed. This is rare, but it can happen, and might actually be inevitable if an elevator is operated for long enough. The failure is caused by a combination of corrosion and repeated stress on the steel so the longer a piston is operated, the more likely this type of failure is to happen . New cylinders (after 1971 or 1972) have a safety feature intended to prevent this type of catastrophic failure. They add another bottom plate inside the cylinder with a small hole in it so that if the actual bottom were to break, the oil would have to get through the small relief hole before leaking out the bottom of the cylinder. These are called double bottom cylinders. The relief hole is sized to limit the “falling” speed of the car to a safe level. If you want a visual representation of this, there is an article from Kone elevators that has a good diagram. Kone is an elevator manufacturer, so the article is also partially a “buy our product to solve this problem” kind of thing. Link to the article: https://www.kone.us/Images/kone-hydraulic-performance-and-safety-upgrades_tcm25-18806.pdf. So if you have a hydraulic elevator that was installed before 1972, you might want to make sure it has be updated with a double bottom cylinder.

Door edge sensor, bumper – maybe the most conspicuous door safety device is the sensor on the edge of the door that keeps the door from closing when something is in the doorway. This is both for accessibility purposes and safety purposes. From the accessibility angle, if someone is still in the doorway when the door is closing, that might mean they need more time to get in or out and the doors should stay open. From the safety angle, if there is someone in the doorway that means the person is in between the car and the landing which is an extremely dangerous place to be if the car were to move. Having the door remain open should prevent the car from moving, and if the car moves anyway the person at least won’t be trapped in between the doors and might have a chance to get out of the way. This type of device is called a safety edge or door edge or door edge detector or something like that. There are a few different types of this mechanism. The older type is a mechanical bumper. When the door hits something, the bumper actuates a switch that causes the door to reopen. These used to be very common but are largely being replaced by newer, and supposedly more reliable technologies (and also that don’t require physical contact with the door to trigger them). Newer safety edges use infrared beams across the door opening. So there is an emitter on one side and a receiver on the other. If the IR beam across the doorway is broken the door won’t close. These started out with one or 2 IR beams across the opening and have evolved to effectively cover the entire door opening with what they call an infrared “light curtain.” There are even what they call 3d light curtains which detect movement or objects immediately outside the doorway. So that keeps the doors from closing when they shouldn’t. We’re also very concerned about keeping the doors closed when they shouldn’t be open.

Door will only open when car door clutch and hoistway door pickup rollers line up which only happens when the car is properly lined up at a floor.

There are also devices that attempt to prevent the car from moving when the doors are open. The governor that I talked about before sometimes performs this function. The governor can be engaged while the doors are open, so if the car moves the governor rope will deploy the safeties to stop the car movement. Another device that is used to prevent the car from moving while the doors are open is called a rope gripper. The rope gripper acts as another type of brake which grand the hoisting ropes to prevent the car from moving.