What could cause a Down Escalator to speed up?

[esca9lante]

Last week at Coors Field in Denver a heavily loaded down-running escalator inexplicably sped up and dumped people in a heap at the bottom, causing many injuries.

Initial investigations reveal no gears or transmission (mechanical) parts were broken or stripped, but it is believed that there was more weight on the escalator than it was designed for (300 pounds per step). So now the investigators are looking for answers, and all the escalators at Coors Field remain deactivated.

I would like to offer this as a possible reason for what happened, and I'd like to hear others' opinions on it.

I assume the motor driving the escalator is a typical induction-type, probably running at 480 volts. In my limited experience with induction motors, if you load one down (my dad had an old table saw that had one of those) to the point where it overloads the motor, it stalls; it doesn't just get slower and slower as more load is applied. So wouldn't it be possible that the exact opposite happened with the motor on the escalator? An induction motor keeps a more-or-less constant speed no matter what the loading is, until the induction is broken. If the motor is being pushed to go faster, it will hang in to the induction for awhile until the pushing load exceeds its ability to hold back (breaks the induction), and then it free-wheels with no speed control at all.

What think you all?

 

[charlie b]

Re: What could cause a Down Escalator to speed up?

Originally posted by esca9lante:An induction motor keeps a more-or-less constant speed no matter what the loading is?.

This is true of a synchronous motor, but not of an induction motor. As you increase the load on an induction motor, it slows down, and the motor current increases. The reduction in speed may not be dramatic enough for it to be noticed by someone just watching (or listening to) the motor, but it happens anyway. The ultimate result of continuing to add more and more load is that you can stop the motor completely. At this point, the current increases to what is called the ?Locked Rotor Current? (LRC or sometimes LRA) value.

But in this case, the motor is not being overloaded. It is being driven by an external force. I believe that the weight of too many people on the escalator exerted a force (i.e., gravitational force) that the gearing (and other mechanical parts) transferred to the shaft of the motor. If you drive an induction motor using an external force, you turn it into a generator. The motor would supply current back through its own branch circuit conductors to the circuit breaker on the power panel. That circuit breaker would not have been designed to include a reverse power trip. As a result, some of the lights in the building received some of their power that day from the motor-that-became-a-generator.

This might have caused some damage to the motor bearings or to some mechanical parts (i.e., from wear and tear in a direction to which they were not accustomed). But my guess is that the damage would be so slight as not to be noticeable.

How can this tragedy be prevented? I am not certain. What I would do next, if I were in charge of the investigation, would be to evaluate the size of the motor (in view of the length of the escalator path and its angle with the horizontal) and the gear ratios, to see if they were matched properly against the design loads.

 

[bphgravity]

Re: What could cause a Down Escalator to speed up?

I believe it was too many overweight people and Gravity.

[ July 08, 2003, 02:33 PM: Message edited by: bphgravity ]

 

[pierre]

Re: What could cause a Down Escalator to speed up?

bphgravity

would you mean - 'being physically heavy' gravity(bphgravity)? I know that was pretty bad.

Pierre

 

[roger]

Re: What could cause a Down Escalator to speed up?

Bryan,

I believe it was too many overweight people and Gravity.

[ July 08, 2003, 02:33 PM: Message edited by: bphgravity ]

now Pierre,

would you mean - 'being physically heavy' gravity(bphgravity)? I know that was pretty bad.

Pierre

these two posts are prime examples as to why we are not making our livings as comediennes.

Well, maybe I still have a chance. (NAH)

Roger

 

[bphgravity]

Re: What could cause a Down Escalator to speed up?

Actually, I wasnt making a joke at all.

Look at recent line of national events.

Escalator failing and hurting several people, Chicago balcony collapsing killing several people, Airplane trying to take off (NC I think) and crashing shortly after lift-off, another airplane trying to take off and crashing shortly after lift-off killing a famous singer/actress.

The bottom line. All cases of overloading and exceeding weight limits.

The problem is that manufacturers and engineers designate a maxixmum load of a structure or peice of equipment at a per person limit that is based on an "average" weight per person.

Studies have shown that the average weight of a person has increased nearly 20 pounds in the last 10 years. I think it is more like 30 pounds. Many airlines are increasing the the per person average and so are many elevator companies.

I cant begin to tell you how many times I see my 300 pound neighbor trying to climb his 150 pound maximum load ladder to pick some oranges. I personally weigh 230 pounds, add in tool belt, nail apron, and cordless drill, the only ladder suitible for me is an A1 300 pound load ladder.

Things are not keeping up with American obesity.

 

[lazorko]

Re: What could cause a Down Escalator to speed up?

Good point. Pass the donuts.

 

[bd150]

Re: What could cause a Down Escalator to speed up?

You are correct The overhauling load tripped the safety on the escalator. If the unit did not have thermal overloads to sense the overcurrent and apply the brake the unit free wheels, hence dumping people off the end. My thought is that the unit did applt the brake but the mass in motion was so much it pulled thru the brake ( common occourence ) The units at Coors fiel need a good like at brake size and maintenance. they worked before why not now ? The units need work. Well I sure hope an army of attornys gety involved and hang the company that built it. Can't be the fact that it was hauling twice the rated load. I hope who ever decided to buy the smaller units got a pay increase.

 

[big jim]

Re: What could cause a Down Escalator to speed up?

The last we heard from the local media is that there was a minor wiring error that caused the brake to be applied a fraction of a second late. That delay allowed the heavily loaded unit to pick up more kinetic energy than the brake could disapate. It does sound to me like the unit was a little too criticaly engineered if it couldn't handle a delay of less than a second.

 

[Guest]

Re: What could cause a Down Escalator to speed up?

bphgravity said:

Chicago balcony collapsing killing several people

It may have been a construction error vs. an engineering error. Not to go too far off topic, but in Kansas City at the Hyatt when several catwalks collapsed it was a construction error. Let's see if I can describe this quickly: There were three levels. They were to be suspended by a continuous length of all-thread. Each catwalk deck would be supported by a nut&washer in the all-thread. The entire load would be supported by the all-thread, but each nut & washer would only carry the load from one catwalk deck. The contractor made a decision to use three pieces of all thread. They were not coupled together. They were staggered from one catwalk deck to the next. I don't have a picture, but the result in making this onsite change was that the entire load off all three catwalk decks was transferred up the staggered all-threads to a single nut. One nut took all the load instead of three nuts sharing the load. The catwalks were never overloaded. They were not under-engineered. They were just built wrong and nobody realized the field change would impact the integrity. Hindsight is 20/20. The forensic investigation quickly pin pointed the failure.

Now, the Chicago deck. Per the UBC tables decks are designed to hold a live and dead load that is hard to overload. Either the deck was unengineered and stamped by an engineer to be built at less than UBC specs, or the deck was built without a permit and the UBC tables were ignored, or the inspector was careless, or time had taken its toll on the structure of the deck. It's not always the fault of the engineering.

../Wayne C.

Here are a couple of links to the KC tragedy:
Skywalk collapse still offers lessons to engineers, inspectors
Hyatt tragedy details