how do lifts work / stop working?

Usually several motors, many,many cables and dozens of switches.

If the power fails. If something happens unexpectedly - eg one sensor says the lift is moving, another says it is stopped. The safest thing for the designer to do is to stop everything, if anything appears abnormal.

I would wait for a lift engineer/ fire brigade to arrive and exit the lift uder their instruction.

Reply to
Palindrome
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i visuallise a lift as a cage; a motor; a cable and a switch. why do they get stuck?

why do they get stuck in between floors?

do you try to exit lifts stuck between floors? or do you fear decapitation/amputation from a sudden motion of the lift?

thanks. k

Reply to
komodore comrade

anything that moves will break, sooner or later

the odds of a failure at any given instant are equal for any given point in the motion range. there are many more points between floors then at designated exit points.

not through the door

or do you fear

been watching a lot of horror / thriller movies lately?

Reply to
TimPerry

Not necessary... Elevator accidents are frequent and common enought to regularly appear in the news:

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also see:

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Beachcomber

Reply to
Beachcomber

How common are they? I've never been involved in one, I don't know anybody who's ever been involved in one, I don't think that anybody I know has ever told me that somebody they know has ever been involved in one. I can't remember the last time I heard a news report about one. There was a report about an engineer working in a shaft who fell to his death several years ago, and another about a child who's fingers were trapped, also some time ago now. Those are the only ones which come to mind.

Obviously such accidents happen, but are they really 'common'?

Reply to
furles

How common are they? I've never been involved in one, I don't know anybody who's ever been involved in one, I don't think that anybody I know has ever told me that somebody they know has ever been involved in one. I can't remember the last time I heard a news report about one. There was a report about an engineer working in a shaft who fell to his death several years ago, and another about a child who's fingers were trapped, also some time ago now. Those are the only ones which come to mind.

Obviously such accidents happen, but are they really 'common'?

Reply to
furles

How common are they? I've never been involved in one, I don't know anybody who's ever been involved in one, I don't think that anybody I know has ever told me that somebody they know has ever been involved in one. I can't remember the last time I heard a news report about one. There was a report about an engineer working in a shaft who fell to his death several years ago, and another about a child who's fingers were trapped, also some time ago now. Those are the only ones which come to mind.

Obviously such accidents happen, but are they really 'common'?

Reply to
furles

There are many different types; all involve rather more than this, mainly in control and safety systems.

Names probably vary in different locations, but the most common now is the traction drive system, where the car is suspended in the shaft by several steel ropes which are wrapped around a drum in the machine room; they are not wound up onto the drum, they are simply wrapped around it, sometimes more than once, and then come off again, so there is always the same amount of cable on the drum. The counterweights are hung on the other end of the cables, at the side of the shaft, so they go down as the car goes up. The cables are driven simply by the friction between them and the drum. The drum is driven by an electric motor, through a gearbox. Moden lifts normally use a.c. motors, controlled electronically, onder ones used d.c. motors powered by rectifiers or motor-generators. Any of these can fail, but as palindrome points out, a lift can be stopped by the failure of a saftey system, of which there are many; door interlocks, overspeed governors etc. The more safety you build into a lift the greater the possibility that it will be stopped by the failure of a safety system. Do you want safety or reliability? If you want more of one then you will get less of the other.

The alternative drum drive system is largely obsolete now; in this the cables suspending the car are actually wound up onto the drum, and there is another drum, with separate cables wound in the opposite direction, for the counterweights. Coal mines normally used this system, as did some of the old lifts on the London Underground; all now removed from service. It tends to be slower than the traction drive system, and the machinary takes more space.

Most electric lifts use one of these two systems, but there are others, such as wheelchair lifts which only move a metre or so to link parts of a building with slightly different floor levels; these tend to be a platform raised by a large screw.

Hydraulic lifts were common in the early days, and seem to be making something of a comeback recently; again, there are several different types. For a short distance the car sometimes sits on top of a piston; pumping fluid into the cylinder under pressure raises the cylinder, and the car. These seem to be popular in shopping centres, with a class-sided shaft, so you can see who they work. This obviously vouldn't be practical over a large distance, so for this an alternative system is used. The car is suspended by cables, as in an electric lift. The cables are connected to what used to be known as a 'multiplying sheave', though I haven't heard that term used recently. Think of lifting a heavy weight by means of a block and tackle system. The rope passes around lots of pulleys in the two blocks. By pulling through a lot of rope you can apply a large force to lift the load a small distance. Now reverse the system; use a hydraulic cylinder and piston to excert a large force to push the pulley blocks apart a short distance. This will take up a much longer length of rope, with less force, and by passing the rope over a pulley at the top of the shaft this can be used to raise the car. There's also a third hydraulic system, where a three-cylinder hydraulic motor, which were used for many porposes, drives a drum, as with an electric lift, but I've only ever seen one of these.

There are quite a few things that can fail, but power or control/safety system failures are more likely than something mechanical.

Sorry about multiple copies of my other post; Internet connection problems this morning, and it thought it hadn't gone through.

Reply to
furles

They are common enough in the US to require extensive yearly safety inspections and the attention of a manufactuers association.

I personally have been a passenger on more than one broken elevator that relied on 1950-60's technology and had not been upgraded properly. The elevator would do things like stop one foot higher than the landing and open the doors or suddenly lurch another 2 feet downward after it had stopped and opened the doors! The repairman found the sensing switches and leveling circuits to be defective. Fortunately no one was injured.

I'm not an expert, but I believe that the current elevator safety technology has mostly eliminated these potential dangers. Still, elevators are very complex systems and things can still go wrong from time to time.

Beachcomber

Reply to
Beachcomber

I don't have a lot of experience of the US, but the equipment looks much the same, and I would think that the inspection and testing procedures would be broadly similar. Surely, the need for inspections is not an indication of how common accidents are. Clearly, putting people into a metal box, and winding it up and down a deep shaft has the *potential* to be dangerous, and the aim of the inspections is to prevent the potential danger from becomming an actual one. If accidents are common, then it would suggest that the inspection procedures are inadequate. If however, as I believe, accidents are rare, despite the potential danger, then this would suggest that the design of elevators, and testing and inspection procedures are generally adequate. Obviously, you can never eliminate all risk. If accidents are rare it does not indicate that there is no need for inspections.

Clearly a lift shouldn't behave like that, and this case certainly sounds serious enough that it should be immediately taken out of service until repaired. Even in this case, which is pretty serious, would not result in anybody falling down a shaft, or being crushed in machinary. Something like a broken leg would be a more likely worst case,

There's nothing wrong with 1950s or '60s technology, there are plenty of lifts of this age still in use, and with a good safety record. I doubt that the modern ones are any safer from the passengers' point of view, though the machine room equipment, both mechanical and electrical, is a different matter, so they probably are safer from the engineers' point of view. They may be less convienient to work on though. The machinary for the old London Underground lifts I mentioned was incredible; they were a 1906 Otis design, with lots of exposed moving parts and 600V d.c. electrics. This was the reason that the last one was taken out of service, and the station it served closed, just a few years ago. The station was little-used, and the cost of new lifts couldn't be justified.

Where I work we had three 50 year old lifts replaced last year. two had been modernised to some extent in the past, the other was in pretty much original condition. They didn't break down that often, but when they did it could take a long time to get repaired, due to the difficulty of obtaining parts. This was one of the reasons for replacing them. Another was that the old lifts were not DDA (Disability Discrimination Act) complient, which the new ones would be. The new ones will take themselves out of service if they detect the slightest thing wrong. For example, one of them recently couldn't close the doors, after a few attempts it gave up, and went OOS. The fault was nothing more than a small piece of chocolate jammed in a door runner. The old lifts would have kept trying, and as soon as somebody noticed the chocolate and removed it, have been working again, or after a few more attempts to close the doors would have broken up the chocolate, and solved the problem. The students (I work in a college) have discovered that if you put a foot in the door for a few seconds you can put a lift out of service, or they do the same thing by holding the doors open for their friends.

The new lifts move quicker than the old ones, but they spend ages at each floor while they announce 'Third floor, doors opening...mind the doors, doors closing [somebody gets their coat sleeve in the way of the door] doors opening...mind the doors, doors closing, going down' etc. This is all part of the DDA thing.

Certainly true, but the system of design, maintenance, inspections etc. usually manages to ensure that when they do, the results are not serious. Being stuck in a lift between floors (it's never actually happened to me, so now I've said that it probably will tomorrow) is obviously inconvienient, and can be unpleasent and distressing, but it's seldom dangerous.

Reply to
furles

Because the hydraulic piston system is so simple and doesn't requirement cable replacement, they are used just about whenever they CAN be used. Six (6) story lifts are routine.

The hole for the piston/cylinder assembly doesn't seem to be much of a problem with modern equipment. The weight of the car is entirely carried from below. The shaft doesn't have be more than a few feet higher than the ceiling of the highest floor.

The only negative is that they tend to be very SLOOOW.

Reply to
John Gilmer

Yep!

I have read about a few accidents. Just about ALL of them are because the door of the car and on the floor opened when the car was not in the right position. In some cases, the car moved after the door was open.

The accidental falled of the car the the basement just never happens.

Slight OT:

When the B-25 bomber flew into the Empire State building toward the end of WWII the support cables for an elevator car were ALL cut. The car fell some 500' to the bottom of the shaft. The operator only had minor injuries and lived to have children and grandchildren.

Reply to
John Gilmer

Because they incorporate numerous redundant safety systems. The preferred failure mode of any of these is to stop the system (rather than drop it to the bottom of the shaft). So, anything that fails or goes out of calibration usually results in the system stopping.

Because stopping at a floor isn't really considered 'getting stuck'.

Not really. But we did have an incident where an elevator stopped above one floor, the occupants pried the doors open and jumped down to the floor. One of them lost his balance and fell back into the shaft beneath the cab and was killed.

Reply to
Paul Hovnanian P.E.

Elevators have braking devices which clamp down on rails running vertically alongside the cab when the cables lose tension. This was Elisha Otis' invention. Undoubtedly, these brakes are not perfect, so the elevator cab will slip. But it will do so at a speed far below terminal velocity making the event quite survivable.

Reply to
Paul Hovnanian P.E.

Its more likely that they appear in the news due to their rarity.

Reply to
Paul Hovnanian P.E.

The primary failure mode of the hydraulic system is quite a bit less eventful than a broken cable. They spring a leak and slowly descend to the bottom floor.

Reply to
Paul Hovnanian P.E.

Actually, one of Otis's first 'safety lifts' is in the Smithsonian. It has a ratchet type of vertical rail on each side, and the pawls are retracted by a leaf spring that is compressed by the weight of the car on the lift cable. Break the cable, the leaf spring relaxes and the pawls engaged the side rails. Later the smooth rail with caliper brakes came along. (maybe the pawls failed sometimes when the car 'dropped' against them???)

daestrom

Reply to
daestrom

The governor cables were cut in that accident preventing the safeties from setting. for full details see

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In part: "Unbeknownst to rescuers, when the hoist and governor cables of one of the elevators had been severed, ropes to other cars had been weakened. Nevertheless, the elevators had to be used to transport those severely injured, including Betty Lou Oliver. As the plane hit, Oliver, an elevator operator, was blown out of her post on the 80th floor and badly burned. After receiving first aid, she was put in another car to go down to an ambulance. As the elevator doors closed, rescue workers heard what sounded like a gunshot but what was, in fact, the snapping of elevator cables weakened by the crash. The car with Oliver inside, now at the 75th floor, plunged to the sub-basement, a fall of over 1,000 feet. Rescuers had to cut a hole in the car to get to the badly injured elevator operator.

"Despite a harrowing experience, Oliver survived, due in large part to the elevator safety devices which served their function, though perhaps not as envisioned. The elevator car safety could not set because the governor cable had been severed by the plane's impact. Therefore, other factors contributed to slowing the elevator and 'cushioning' its fall. As the elevator fell, the compensating cables, hanging from beneath the car, piled up in the pit and acted as a coiled spring, slowing the elevator. Also, the hatchway was of a 'high-pressure' design, with minimum clearance around the car. In such a small space, the air was compressed under the falling elevator. With such a tight fit of the car in the hatchway, the trapped air created an air cushion in the lower portion of the shaft -- thereby further slowing the elevator car and allowing its occupant to survive.

Reply to
Stephen B.

I think the highest I've seen are five story ones; there are a pair of them near to the main entrance of the Science Museum, in London.

The hole might not be, but how do they transport and install the piston? Does it come in sections that are screwed together? I've never been able to see a join in the ones I've looked at.

That's certainly another advantage; also no need to move heavy machinery into a machine room at the top of the shaft.

Not really too much of a problem for a short rise of just a few floors.

They're also easy to move in the event of a failure; just open a valve slightly and the thing comes down slowly under gravity.

Reply to
furles

A broken cable, or even several, shouldn't be too eventful either, though it will take longer to get people out.

Reply to
furles

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