Safety wire spec'ing

[fireball40k]

Could someone please confirm the correct way to specify a safety wire for a lantern?

 

Do you apply a 5:1 SF and multiply the lantern weight by 5?

 

Thanks

[Snailtrail]

Most safety bonds will be marked with a Safe Working Load which does what it says on the tin. If you have a bond which is unmarked then your guess is a good as mine as to what load that can take, without distruction testing....

 

The Safe Working Load (sometimes marked as WLL - Working Load Limit) is determined by the maximum breaking strain divided by a factor of safety.

 

Hope this helps,

 

Sam

[Seano]

Could someone please confirm the correct way to specify a safety wire for a lantern?

Do you apply a 5:1 SF and multiply the lantern weight by 5?

 

There is no 'correct' way AFAIK. Other than to apply your judgement and experience, risk assessment, blah blah (yawn).

 

You can't simply multiply the weight of the fixture by a 5:1 FoS though, since if the safety bond were to be loaded the (transient) load on it would be a fair bit higher than that. You'd be dealing with an (er... for the want of a better term) 'impact' load as the fixture free-falls for a (hopefully) short distance and then (again - hopefully) comes to a halt over a rather shorter distance.

 

That load isn't easy to predict - it depends on how far the fixture free-falls, and then how abruptly it comes to a halt. If you wanted to be scientific, you'd need to apply your 5:1 FoS to the 'worst case' load of the furthest possible free-fall followed by the most abrupt possible stop. Newton's second law, F=ma, you know 'm' but have, at best, an educated guess of what 'a' is going to be.

 

In effect you'd be applying a much higher FoS to the weight of the fixture.

 

I think my advice would be to buy safety bonds made for the purpose from a reputable supplier, and let them worry about the spec. eg: clicky

 

I suppose if you wanted to, you could 'reverse engineer' what is specced there to determine what FoS they're using (I cant be bothered to work it out, but its a lot higher than 5:1). Bear in mind though, from a theoretical arse-covering point of view, that if you were ever asked to show your workings "its what supplier X seems to do" probably wouldn't impress anyone.

 

Most safety bonds will be marked with a Safe Working Load which does what it says on the tin. If you have a bond which is unmarked then your guess is a good as mine as to what load that can take, without distruction testing....

 

The problem being that they don't come in a tin. Working out what load a safety bond can take is easy, the difficult part is working out the load you need it to take.

 

Newton's second law, F=ma. You know 'm' (the mass of the fixture) but have, at best, an educated guess at what 'a' will be.

(Unlike in the case of using a SWR to 'hang' something statically, where 'a' is simply 'g' - ie: 9.81 meters per square second.)

 

The use of 'SWL' or 'WLL' in the case of a safety bond is slightly misleading (and if you wanted to be really pedantic about it, slightly wrong).

What they're actually specifying is the maximum weight of fixture you should use it with, which really isn't quite the same thing because the weight of the fixture is not the load the safety bond has to be able to cope with.

[Brian]

The definitive answer is...

 

With the end of the secondary suspension remote from the luminaire securely fixed, the luminaire is allowed to hang freely by means of the secondary suspension alone. The luminaire is raised in its hanging mode a distance of 300mm vertically and allowed to fall freely. This test is made 30 times.

 

The secondary suspension shall not fail and no part of the luminaire shall fall.

[Richard the chandler]

Just to throw some additional 2p's into the pot.

 

 

Working from the other way round as a wire assembly manufacturer.

 

In layman's terms you get 90% efficiency with the terminations and then that figure has the 5:1 factor of safety applied to it.

 

Interestingly, and on a slight side point Nicopress are the only manufacturers of these sorts of bits that rate their ferrules to 100% of the load of the wire.

 

 

 

Richard at Flints.co.uk

[ianl]

having just bought some "safety bonds" and some "accesory bonds" both made by doughty but sold by many (flints, cpc, etc) with the suppliers name on the swl heatshrink

 

once purchased there is nothing to state this is an accesory bond and this is a safety bond, the swl labels are almost identical and do not mention their "purpose"

 

however:

 

"accesory bond" made with 2mm wire SWL 20kg

"safety bond" made with 3mm wire SWL 15kg

 

and a "general purpose bond" made with 3mm wire has a swl of 100kg

 

obviously, different purposes, different safety factor; however to the user a bond is a bond

[ramdram]

Ref the point PE made about the drop test; if anyone can recall their "O" level physics they will know that acceleration due to gravity is 32ft per second squared. (nominal figure and in old money). Applying the calcs which I can't recall (but google can) an object falling from a millimetre onto a mythical plane, say, will exert a force on that plane so to speak of near enough the weight (for ease of explanation) of the lantern.

 

You might, if arsed, google up potential energy and kinetic energy; might be of interest?

 

If the lantern drops from 300 millimetres the force it exerts on the said plane will be greater than the static weight of the lantern. If dropped from several metres the force of the impact will be a lot greater owing to the acceleration from gravity.

 

(You may have heard in school of the effect an old penny, falling sideways, would have on a human skull if dropped off the top of the Forth Railway Bridge...it would enter the skull with comparative ease.

 

Think also (for ease of explanation) of trying to press a 6" nail into a block of timber...extremely difficult. Yet, when walloped a few times the hammer head exerts enough force to drive the nail in fairly easily...exchange the hammer for a 14lb sledge and the task is even easier (ignoring the fact it would not be as easy to wield a sledge).

 

So, if you are going to suspend a lantern from a bar then it follows that the safety chain or wire should be as "tight" or short as practical so as to allow full movement of the yolk swivelling on the end of the clamp. This helps to defeat the acceleration due to gravity thing.

 

As for this 5:1 ratio the best way to cover this is to simply buy a safety wire/chain with a much higher "strain" figure (or WLL or SWL, you will have to google for the current expression and definition). I would be buying, for a 5kg lantern, something much higher (see table blow). Plus if I have a load of lanterns (all of different weights) then I would go for the one size fits all approach to avoid having to search for the "correct" one. If you look here (usual disclaimers):

 

http://www.stagepoint.co.uk/Sales_rigging.htm#Safety%20Bonds%20&%20Chains

 

then you will see this section:

 

Safety Bonds & Chains

 

2mm x 600mm c/w M6 snap carbine hook - SWL 20KG (silver / black) £4.86

4mm x 600mm c/w M8 screwgate & carbine hook - SWL 36KG (silver / black) £12.33

3mm x 600mm soft loops SWL 100Kg £2.21

4mm x 600mm soft loops SWL 175Kg £3.48

6mm x 1000mm soft loops SWL 400Kg £5.02

Safety chain 600mm black enamelled c/w 50mm spring hook on one end £2.43

Safety chain 600mm black enamelled c/w 50mm spring hook on each end £3.66

 

 

Another word of caution about the length of the drop you allow the lantern to fall before it snubs against the safety. It may be that you might be looking at an elderly alli' Parcan. Alli' does not rust but it can corrode. Who could say that the force of a long fall would not tear out the yolk bearers and leave the yolk "safe" yet the body of the lantern some few metres below?

 

If you have ever lifted an ancient Cantata onto a bar then you can imagine the force that could exert from only the said 300mms.

 

And, a comment for the less experienced readers sort of thing, more modern and not el cheapo lanterns have "hardpoints"...so always attach your safety bond to that cf a yolk.

[Richard the chandler]

The difference between them really is the intended usage which often then denotes the fittings that are put together onto the assembly and are thus rated alongside the actual wire aspect of it. So in common usage an 'accessory bond' has a crabine snap hook on it and is thus used for accessories such as barn doors and the like whereas an 'safety bond' has a karabiner.

 

As both a retailer and manufacturer of these things we have to be very careful on this sort of thing to comply with our ISO:9001 and LEEA and so for instance if you just purchased a drum of wire of us then the certification wouldn't have anything other than the breaking load of the wire on it, not a SWL with a safety factor applied.

 

 

Simply put iunless its come supplied on a piece of purchased or hired in kit then its up to the end user to know the correct usage of the item, but the information is all on the label.

 

 

To be honest it would be more useful if all of us manufacturers actually tried to add more descriptive info onto them if we know what they are going to be used for. We frequently make up inhouse custom rigging for shows and add extra labels that say things like 'downstage track' or 'upstage window left' etc but I'm sure that if you purchased a new moving head then it would be quite useful to have the safety's marked up with an additional label saying 'MAC 250 SAFETY' or something like that.

 

 

 

 

having just bought some "safety bonds" and some "accesory bonds" both made by doughty but sold by many (flints, cpc, etc) with the suppliers name on the swl heatshrink

 

once purchased there is nothing to state this is an accesory bond and this is a safety bond, the swl labels are almost identical and do not mention their "purpose"

 

however:

 

"accesory bond" made with 2mm wire SWL 20kg

"safety bond" made with 3mm wire SWL 15kg

 

and a "general purpose bond" made with 3mm wire has a swl of 100kg

 

obviously, different purposes, different safety factor; however to the user a bond is a bond

[Andrew Edwards]

Another thing to consider is that primary suspensions points, whether that be the hook clamp, nut bolt set or yolk don't just fail randomly. The most likely cause of failure is being hit by scenery etc. In the case of continuously applied force by the scenery the weight of the fixture is largely irrelevant. The forces sufficient to snap the clamp, nut/bolt or yoke would probably snap the safety bond too.

 

I have always worked to get a bond at least twice the WLL as that is what has been supplied with all my bar ready fixtures I have purchased over the years. These bonds usually have a minimum breaking load of about 10 times the WLL.

 

I also ensure that there is minimal slack in the bond where possible as the amount of time it is in motion would be the primary factor rather than Newtons second law. It would be the velocity at play here and we are looking at Kinetic Energy.

 

*tries to remember A level physics*

 

So we have Newton's Second law of Force = mass x acceleration but we need to know the mass of the fixture. If we plug W=mg (the formula for finding weight and I'll get back to g later) into Newton's second law we get the following.

 

W=mg swapped round to make m=W/g

 

F=ma substitue m for W/g giving

 

F=(w/g)a

 

If we say that a is in fact g (g or more precisely g₀ is gravity in vacuum near the surface of the earth) we get

 

F=(w/g)g and the g cancels out gving F=w.

 

Let's say that a fixture falls for 1 second before snatching on the bond. Assuming no other factors this means that the fixture would be travelling at less than 9.8m/s. Momentum is mass x velocity so the kinetic energy of the fixture after 1s would be less than 10 times than that at rest in this example.

 

too much physics for Friday afternoon...

 

Edit: 2 previous post appeared whilst I was typing (was doing this at the same time as something else)

[Brian]

Let's say that a fixture falls for 1 second before snatching on the bond.

Just how long are your safety bonds? I'm sure they're not 9.8m long.

[Peter Tovey]

2 points:

 

1) Eggs have yolks. Lanterns have yokes.

 

2) You can't just substitute g into F=ma to determine the force on a safety bond if it were to catch a lantern. The figure for maximum acceleration (or deceleration in this case) comes from the initial speed when the bond starts to arrest the fall, and the length of time it takes the bond to completely arrest the fall. This is extremely unlikely to be linear, is a second order effect, and depends on the material properties of the bond, the lantern, and all the suspension points.

 

And an answer:

 

Either ask an engineer to work it out for you and give you the specifications on paper, OR - call Flints, Doughty, SLX, Rope Assemblies, whoever. Tell them the mass of your fixture and the length of bond you want, and let them work out what to make it from. They can label the bond for a maximum fixture mass and send you certification with the bonds.

 

And a piece of advice:

 

Fix your bonds to the fixtures they are intended for. That way they can't be used on fixtures beyond their design capacity, and they don't go missing. SLX have a very neat way of doing this on their hire stock using an (unrated) brass ferrule to hold the bond back to itself after being looped around the yoke or safety point. Just be sure to place this crimp near the carbine / carbiner end of the bond so that numpties can't use this end to go around a bar and choke back to itself rather than taking it back to the eye on the other end of the bond. (Choking safety bonds is generally a bad idea in any circumstance, but spectacularly bad in this situation.)

 

Pete

[brainwave-generator]

Fix your bonds to the fixtures they are intended for. That way they can't be used on fixtures beyond their design capacity, and they don't go missing. SLX have a very neat way of doing this on their hire stock using an (unrated) brass ferrule to hold the bond back to itself after being looped around the yoke or safety point. Just be sure to place this crimp near the carbine / carbiner end of the bond so that numpties can't use this end to go around a bar and choke back to itself rather than taking it back to the eye on the other end of the bond. (Choking safety bonds is generally a bad idea in any circumstance, but spectacularly bad in this situation.)

 

Definitely want to mirror this advice, it's very good to use. I can think of a lot of places I've been (including major stuff!) where the safeties have just been considered as generic 'safeties' and not designated to a particular fixture. The issue here as I'm sure you'll all appreciate, is that a safety supplied for use on a PAR64 may not be suitable for use on a Varilite VL3500 (the latter being significantly heaver than the former).

 

In places where it is impractical to leave the safety attached to the fixture (IE if it spends half it's life in a flight case and the safety stops the lid shutting or whatever), in one venue I worked in we had a safety colour coding system. Whilst yes, you could generally look at the diameter of the wire to see which were the bigger safeties, we had a simple red yellow green safety colour system, and corresponding tape wrapped in a convenient place on the fixture. Everyone knows Red comes above Green on a traffic light system, so Red came above it in rating too. Thus, you could use a Red, Yellow or Green safety on a Green fixture, but only use a Red safety on a Red fixture.

 

Finally, one final piece of advice. It is generally good practice to be able to trace every individual piece of safety kit in your use. For inspection history, ensuring you can find it if it needs to be isolated etc... it's generally good to be able to individually label all your safeties. I know it sounds like a right pain, but such is life. Good quality vinyl labels are not too expensive, and depending on how much stuff you look after, numbers might be the easiest thing, or a barcode. Labels are good anyway as you can specify what it is, it's SWL, it's individual ID number, and the name of the theatre it belongs to (to prevent it being packed by touring productions by accident), a grade or quality marking (things like CE marking if necessary), and so on.

[ramdram]

Oh well, the yokes on moi. But shirley that should be carabiner?

 

Anyway, a thought occurs; if you were to wrap the safety wire around the bar and yoke twice, say, to lose the slack, minimise the drop, avoid huge acceleration etc etc, would that imply the safety rating is/could be considered at twice the given figure for a single wire?

 

I base this presumption on seeing a diagram in the Kupo catalogue (last year's Plasa) where an endless round sling, or spanset, rated at 2t is able to support 4t if used in vertical basket configuration. Naturally, this assumes I have understood the figures in the catalogue correctly.

[Nicktaylor]

"could be considered at twice the given figure for a single wire"

 

I would have thought not, as you still rely on the single carabiner on the end as the absolute stop. I agree though losing the surplus is what I would do to minimise the acceleration due to gravity bit of the maths people have alluded to!

[Ashley R]

(Choking safety bonds is generally a bad idea in any circumstance, but spectacularly bad in this situation.)

 

Choking as in attaching the carabiner back onto the cable, instead of onto the loop on the end?

 

Wished I could have used my nice thick 50kg rated safety's on my Entours, but alas the cable doesn't fit through the hole allowed for the safety cable....