What happens to the load when you hit an E-stop?

[daiwilliamsuk]

When using Chain Hoists how do you calculate the imposed load at the top point (and on the piece you're lifting) when an E-stop is hit or there is a sudden loss of power?

 

My worry here is not the hoists themselves (which as I understand it have this factored into their designed SWL) but rather the top rigging and scenery piece.

 

Does anyone know of any data which shows how quickly a hoist brakes in the event of a power failure? Given this information and a known speed and load it should be possible to calculate the imposed loads.

 

The background to this question: I have a 10t piece of scenery, with 20 performers sitting on top of it (so another 2t by the time you factor in a bit for "live" loads). It is lifted by 6 of Liftket 2.5t hoists running at 8m/min and controlled by a computer system (TBC - probably Kinesys). I was planning to use 4.75t load sensing shackles above the hoists. However, as the shackles are based on a 5:1 SF I need to de-rate them to 2.375t to keep a 10:1 ratio (in line with our performer flying risk assessment).

 

It seems to me that in the "worst case" situation of a power cut or someone hitting an e-stop the imposed loads at the top could easily exceed the new SWL of the shackles.

 

Any insight appreciated!

[paulears]

Erm. Isn't an e-stop the same as an ordinary stop in terms of stop and lock speed?

[aidso]

surely yes paule. cause the brake releases under power on all motors as per regs. It does that just incase of power failure. All lodestar and Verlinde systems work this way, that when e-stop is hit it breaks the mains coming into the controller or break the output control in lodestars. Which is the same as a normal stop, unless you are using veri speed motors and have a wind down programmed in to reduce the sudden stop.

[Seano]

When using Chain Hoists how do you calculate the imposed load at the top point (and on the piece you're lifting) when an E-stop is hit or there is a sudden loss of power?

<snippage>

Does anyone know of any data which shows how quickly a hoist brakes in the event of a power failure? Given this information and a known speed and load it should be possible to calculate the imposed loads.

 

I'm uncertain, but honestly I don't think you can make a meaningful calculation. It depends on too many variables that you can't accurately predict. I suspect it will be a function more of the 'elasticity' of your top rigging (and the supporting structure) than of the motors themselves. If your motors are rigged at different points along supporting beams, the deflection of those beams will vary somewhat from point to point, and therefore the additional dynamic load imposed by a 'crash' stop is likely to be different for each motor.

 

I think the best you can hope for is a 'rule of thumb' (which I can't help you out with I'm afraid). I think you're right to suspect that the transient load will (briefly) exceed 2.375t though - a transient additional load of, say, 25% doesn't seem overly pessimistic.

 

12t seems quite a lot to lift on 6x2.5t motors, does your set design permit only 6 points or might it be possible to go to 8 motors instead?

 

It seems to me there are two ways to go:

If you regard a 'crash' stop as an 'unplanned' event which you've taken steps to prevent, you could argue that that is precisely what your additional safety factor is designed to accommodate. (And therefore exceeding your extra-cautious SWL for a fraction of a second in the worst case is ok.)

Or, if you regard a 'crash' stop as something which is inevitably going to happen at some point, you're going to need beefier load cells and 6.5t shackles.

 

I would probably lean toward option a myself, but then of course its not my gig. ;)

 

Erm. Isn't an e-stop the same as an ordinary stop in terms of stop and lock speed?

For a bog standard model L lodestar, yes it is.

Otherwise, for a more sophisticated system where speeds are ramped up and down - nope.

Its the difference between pulling up at a junction in your car and standing on the brakes when someone steps out in front of you.

 

I've had a couple of interesting conversations with industrial types about e-stops. With large machines and processes, its actually quite a tricky engineering question deciding just how quickly an e-stop should stop the machine. Obviously you want everything to stop as soon as possible, but trying to stop it *too* quickly has dangers of its own - infinite deceleration requires infinite force.

 

unless you are using veri speed motors

The OP made it very clear in his post - he is.

[tokm]

A little side point that comes to mind.. might not be relevant.

 

Can the structure/piece of set withstand the various forces placed upon it from being stopped suddenly? The tale of a mothergrid with a large rig hung bellow it, having its motors 'bumped' and then failing due to forces presented on them from the sudden stop comes to mind..

 

T

[Seano]

The tale of a mothergrid with a large rig hung bellow it, having its motors 'bumped' and then failing due to forces presented on them from the sudden stop comes to mind..

 

Sounds a little unlikely. What tale is this? True story? Where/when did it happen?

[dbuckley]

Alternative possibility: In most juristictions there are stringent rules about E-Stop systems, but there are variations possible if the actual act of E-Stopping creates danger.

 

So..... Wire the E-Stop buttons to a PLC. When the E-Stop is triggered it commands the existing control system to do as rapid speed-down as is considered safe, say half a second. At 0.6 seconds after E-Stop initiation, the PLC opens the power to the system as a conventional E-Stop would do. Thus you get a safe "slow" to halt, with a backup if that fails.

 

Of course, when I say PLC I'm not talking a off the shelf micro-PLC of which I am so fond, but something which is rated (and certified) for just this type of job, by someone like Pilz, or, well, Pilz.

[tokm]

Sounds a little unlikely. What tale is this? True story? Where/when did it happen?

Will get clarification and PM you about it. T

[daiwilliamsuk]

Thank you for your replies. That's all helpful stuff

 

Seano: Yes I see your point on calculation.

 

The theatre is 4 storeys below ground with a swimming pool sitting above it. There was approx 60mm deflection in the centre of the grid (60m span) when they filled the pool - effectively pre-loading the structure by a much greater degree than I will be. Each attachment is a welded hard point directly to a node point on the steel work of the the building and the steel work itself is arranged in such a way as to make each point structurally similar. There would only be a very short SWR between point and motor. So losses due to elasticity should be minimal (which makes it worse for me), although there will be some, hard to quantify, flex in the scenery piece itself.

 

Even given all that, you probably can't get a accurate number.

 

In the past I've always worked on a +25% rule of thumb for chain hoists. I believe that's a fairly accepted figure which takes in to account the loads generated by lifting using a standard "dumb" hoist. Which, as has been pointed out, effectively "E-Stops" every time it stops. For the bulk of rigging using 1ton chain hoists on 3.25t shackles and 2t spansets that works fine. I read somewhere (possibly this forum) that that figure is based on a standard Lodestar - I'm assuming at 4m/min. More out of interest than anything, is anyone aware if this figure is greatly affected by using faster motors/different brands?

 

But, anyway, in this case I need to be a little more in-depth for a couple of reasons.

 

1) I'm closer to the absolute SWL of the kit. Both the motors, the top rigging and the scenery piece. I'd love to use 8 motors but unfortunate budget doesn't allow it (for the moment - if I can't work out a way to do this with whats available I'm sure the money will be "found" but I'd see that as a bit of a cop out).

 

2) The project is in Singapore, where they are currently going through a tightening up of their lifting regs. At the moment they are in the situation we were maybe 10-15 years ago where a lot of the enabling legislation has been passed but very few codes of conduct or specific regulations have been written. This means a lot of construction industry stuff is being forced on the entertainment sector. To get round these regs I have to put together a proposal and get it authorized by a 3rd party registered with their version of the HSE. To better argue my case I was wondering if there was some data I could use.

 

I would probably lean toward option a myself, but then of course its not my gig. :P

 

I actually agree with you. But need to be a little over cautious for the above reasons.

 

Or, if you regard a 'crash' stop as something which is inevitably going to happen at some point, you're going to need beefier load cells and 6.5t shackles.

 

Yes , I guess this is the route I need to go down. I was trying to get round it so I could use Libra shackle pins with Kinesys control and have one fully integrated system from one manufacturer. (with all the cost and support savings that come with it

 

So..... Wire the E-Stop buttons to a PLC. When the E-Stop is triggered it commands the existing control system to do as rapid speed-down as is considered safe, say half a second. At 0.6 seconds after E-Stop initiation, the PLC opens the power to the system as a conventional E-Stop would do. Thus you get a safe "slow" to halt, with a backup if that fails.

 

Yes, that would be a way out of it. Not sure if their implementation is the same as yours but Chainmaster do a system with precisely that feature. It's fully BGV-C1/SIL3 compliant as well which should be accepted in most jurisdictions. Its very expensive though.

[dbuckley]

Its very expensive though.

Relative term.

 

I hear people talking about half a dozen chain motors, VFD driven with a computer, and literally tonnes of truss and I think, f$$k, that must cost a fortune.... Whereas Pilz safety rated kit is not cheap exactly, but to me it seems so in relation to that collection of heavy metal...

 

As you've name dropped the SIL term you're clearly on the ball with this stuff :P

[daiwilliamsuk]

Yes a relative term.

 

In comparison to the show costs and the consequence if something goes wrong we're not talking ridiculous numbers.

 

But its still a substantial premium over the Kinesys system I'm looking at, and has a lot of "features" I simply don't need. There's no need for joysticks and touch screens if the system is essentially going to be running the same 4 cues for the next 26mths (the projected life of the show).

 

For the record I'm no systems guy, so, although I understand the concepts behind SIL, I can't claim to be "on the ball" on the actual engineering issues.

 

I've heard that a version of BGV-C1 is coming as a BS EN standard, so I'm guessing its something we're all going to have to be aware of soon.....

[Seano]

I should prolly preface my comments with a disclaimer: I know bugger all about automation.

I have a fair bit of experience of hanging stuff up in more or less the right place. But so far I've always had enough spare capacity not to have to worry about the transient loads associated with things that whizz and whirr and jiggle up and down, and long may that continue.

 

So..... Wire the E-Stop buttons to a PLC. When the E-Stop is triggered it commands the existing control system to do as rapid speed-down as is considered safe, say half a second. At 0.6 seconds after E-Stop initiation, the PLC opens the power to the system as a conventional E-Stop would do. Thus you get a safe "slow" to halt, with a backup if that fails.

 

Interesting. I guess besides a lower transient load another advantage to that approach would be a more accurate prediction of what the transient is likely to be like using nice simple maths. At least if you take a worst case of the load descending at full tilt, its just F=ma.

 

For example. Lets say your maximum allowable transient load is 20% on top of the static weight of the piece.

That implies maximum acceleration (deceleration) of 0.2 x g = 1.96 m/square second

8 m/minute = 0.133 m/s so if you assume a linear 'ramp down' to a halt, that implies stopping over 0.133/1.96 = 0.07s

 

I've no idea what Kinesys is capable of - would it be possible to ramp down to a halt over (say) 0.1s? (giving a 14% 'overload')

 

Obviously that's very simplistic, you'd be pretty silly to take my word for it - but it does seem to me that it would cover the biggest bump in the worst case (followed by a lot of smaller peaks and troughs as everything settles down back into a 'steady state').

 

Of course, thats still no help in the case of a power cut.

 

Even given all that, you probably can't get a accurate number.

 

In the past I've always worked on a +25% rule of thumb for chain hoists. I believe that's a fairly accepted figure which takes in to account the loads generated by lifting using a standard "dumb" hoist. <snip> I read somewhere (possibly this forum) that that figure is based on a standard Lodestar - I'm assuming at 4m/min. More out of interest than anything, is anyone aware if this figure is greatly affected by using faster motors/different brands?

 

I can't get an accurate number, certainly. :P That doesn't imply no one can. Even if it can't be done by calculation, there must be empirical data out there.

All you'd have to do is hang up a test weight on a suitably solid structure* with the right kind of load sensing and actually try it - surely, somebody must have done it.

* - Really, really easy to achieve in a test rig - it could be knocked up in any decent staging company's yard in about 1/2hr.

 

Do you have suppliers quoting for the Kinesys (or whatever) yet? Might they be able to tell you something?

 

Regarding the 25% rule of thumb for 'dumb' hoists - yes, I've always worked to that too. I'm not certain, but I think that number was first quoted to me at a Lodestar 'Motor School' course some years ago. And yes, as far as I'm aware that applies to a 'standard' Lodestar. (Model L - 4m/min). Its just intuition, but I'd expect it to be higher for faster hoists, and broadly similar for different brands. Not something I've ever had to look into in more depth, since faster hoists are generally only used on lighter loads. (Big fancy Kinesys set waggling systems aside. ;))

 

In the case of PA and video systems especially, things are probably made much worse by the habit of certain techs of adjusting things in rapid bumps. <click><click><click> *Hate it* when they do that - It always seems they're hitting the resonant frequency of the roof, and a mental picture of the Tacoma Narrows bridge springs to mind.

 

But, anyway, in this case I need to be a little more in-depth for a couple of reasons.

Interesting. And *way* outside of my experience. I'd love to hear what you've learned when you get it all sorted out.

 

I've heard that a version of BGV-C1 is coming as a BS EN standard, so I'm guessing its something we're all going to have to be aware of soon.....

Already with us, I believe, in the form of BS7906: Part 1 - Category A

 

'Aware of' is probably a relative term though - its not as if BS standards are freely published and widely distributed or anything - they're *expensive* and invariably each one refers to a dozen more.

Its one thing to be aware that a standard exists, and something else again to actually get your hands on it and read the small print.

[JimWebber]

So..... Wire the E-Stop buttons to a PLC. When the E-Stop is triggered it commands the existing control system to do as rapid speed-down as is considered safe, say half a second. At 0.6 seconds after E-Stop initiation, the PLC opens the power to the system as a conventional E-Stop would do. Thus you get a safe "slow" to halt, with a backup if that fails.

 

 

I AM NOT A RIGGER, NEVER HAVE BEEN AND NEVER WILL BE.

Now I am not in a position to comment on rigging this behmoth per se, but another argument for using dbuckley's solution above may be performer safety.

 

If you have talent (And I do use the word fairly loosely), doing "Things" on said item of scenery, and an emergency stop event did occur, the sudden stop may cause someone to lose footing / grip.

 

In fact, I would Imagine that the risks of a sudden stop are actually greater then a standard controlled stop. - The only reason I can see off the top of my head why you would want an emergency stop is if a member of crew gets intimate with the winches.

 

 

I am not offering that comment specifically to your situation, but in possible similar situations in outside industry.

 

I AM NOT A RIGGER, NEVER HAVE BEEN AND NEVER WILL BE.

 

Jim

[Seano]

In fact, I would Imagine that the risks of a sudden stop are actually greater then a standard controlled stop.

Of course they are, but not as great as the risk of coming slowly to a halt when you really need to stop right NOW.

A 'soft' E-stop would most likely still be pretty abrupt compared to the routine cue - a fraction of a second rather than several seconds, and in any case there is still the possibility of a power cut to consider.

 

Issues concerning the safety of performers on a moving set piece need to be addressed regardless, through the usual route of risk assessment and yadda yadda yadda. That's probably not the responsibility of the OP though, and so is somewhat OT as far as this thread is concerned.

 

The only reason I can see off the top of my head why you would want an emergency stop is if a member of crew gets intimate with the winches.

I can think of quite a few other possibilities. They're not winches by the way - the risk of someone getting tangled (and mangled) in the machinery are actually rather less with chain hoists, particularly if they're rigged 'motor up' as they generally are for use with anything other than lighting trusses.

 

But that's beside the point - the main reason to have an E-stop system is to deal with the emergencies you couldn't see coming.

[dbuckley]

But that's beside the point - the main reason to have an E-stop system is to deal with the emergencies you couldn't see coming.

As another non-rigger - and a wimp who doesn't do heights so never will be up there - this is actually a really important facet of this sort of problem.

 

If there are forseeable issues then they should be dealt with by standard approaches. For example if someone needs to be in the right place for a movement to be safe, then a manual deadmans switch, or a sensor that detects the presence of the required person can cope happily with this forseen event, and all the right "stuff" to make this happen safely is available from your local Pilz vendor. Industrial machinary uses this approach all the time; its all very well having an E-Stop for an industrial press, but if someone gets a body part stamped by said press then H&S will be all over you like a rash wanting to know where the interlocks were that would have stopped the punch from engaging.

 

E-Stops are reserved for that "0h f__k" moment when something that you could never have forseen happening actually takes your breath away because it really did just happen and stuff needs to stop and stop now.