Safety wire spec'ing

[ramdram]

Interesting point on carabiners. I can just about recall seeing a load of them at home(old man went climbing)and noting some were simply snap connectors and some had a threaded sleeve. IIRC the load rating was a fair bit higher when the crab' was closed and the sleeve tightened up.

 

It occurs to me that the rating on a crab' would/should be higher than the cable anyway, otherwise pointless to use it.

 

I suppose the only real way to know would be to the experiment thing and sacrifice a few items.

 

And whilst musing on this topic and reading the posts above you might wonder why a lantern manufacturer could not include a proper rated safety already attached to the lantern, during manufacture. Cf the cost of the unit the safety bond would be trivial, is always attached, ergo no excuse for not attaching the safety seconds after hooking the clamp on the bar.

 

As for the choking issue surely it is not beyond the wit of the designer to engineer in two hard points?

 

(Not having see every lantern made possibly this is the case in some instances???)

[TomM]

 

And whilst musing on this topic and reading the posts above you might wonder why a lantern manufacturer could not include a proper rated safety already attached to the lantern, during manufacture. Cf the cost of the unit the safety bond would be trivial, is always attached, ergo no excuse for not attaching the safety seconds after hooking the clamp on the bar.

 

As for the choking issue surely it is not beyond the wit of the designer to engineer in two hard points?

 

(Not having see every lantern made possibly this is the case in some instances???)

 

I believe some models of Selecon fresnels do.

[ramdram]

I was tempted to say spot on TomM...but I didn't.

 

I note from the Selecon photo that the crab'on the safety is in the "choke position" which, according to this brochure I've been reading reckons that this "choke position" lowers the rating to 80% of "a" quoted rated load.

 

Had a very quick google for other makes but Selecon was the only make thus equipped.

[dbuckley]

The Seleaon Acclaim fresnel and PC do indeed have an integrated safety wire feature. And very nice it is too. There is a knob on the top which allows one to adjust the length of the safety line, which solves the problem of the too long wire needed to be wrapped round the br several times. It's also part of the lantern, so it gives more coverage than the normal loop of wire through the yoke.

 

http://www.flash-light.org.uk/ccp51/media/images/product_detail/1013.jpg

[Ashley R]

You forgot the Acclaim Profiles as well! (But its just crimped through the safety eye built into the fixture on the back, none of this fancy adjustable length safety's)

 

So it's bad to choke a cable as shown in the pic above?

[Peter Tovey]

So it's bad to choke a cable as shown in the pic above?

 

Well, it's better than not attaching the safety at all...

 

If the bond is choked like that, then there are 2 effects. One is that if the bond were used to arrest a load, there would be a greater maximum force exerted on the wire as it's all taken by one CSA of wire and not by 2 in parallel - and this may well not be the mode of use for which the bond was designed and rated. Realistically the chance of this actually making any difference is vanishingly small if you're using the right bonds for the fixtures. The second and more significant effect is that if a bond is choked like in the picture above, then in arresting a fall the connector (carbine hook, carabiner, whatever) is going to be subject to a dynamic side load, which is extremely unlikely to be a mode for which it is rated and certainly one in which it will be considerably less strong and thus more likely to fail.

 

And if you choke a safety bond which is crimped around a lantern in the manner I described in an earlier post, a la SLX, then the entire safety assembly relies on an unrated copper ferrule (or even a cable tie) which is just designed to keep the bond attached to the lantern, and which WILL fail. Hence the importance of having this crimp at the connector end of the bond, and leaving only the eye end of the bond free to wrap around your anchor point and come back to the connector at the other end.

 

Pete

[brainwave-generator]

With all this choking/wrapping/shortening etc business there is a really easy way to put it.

 

The ideal situation with a safety, contrary to some belief, is that the object DOES NOT MOVE.

 

If you can get it so that the safety is almost under load, but just not quite, so that should the main suspension fail, it simply moves the load onto the safety, that is ideal. A lot of people imagine that a safety should work by catching an already falling fixture. This is not the case at all. The secondary suspension should be exactly that - a suspension, just second to the main (primary) suspension.

 

However, for this reason it is important that you check your lighting fixtures reasonably often, every 6 months should be good, or at least annually. If your safety was attached like I've said above, the problem is that you may not notice that it's primary suspension (ie the clamp) had failed at all, it may 'catch' it so quickly that the main suspension appears to be intact, even if it has failed.

 

With a choke, or a single wrap around a truss with a load of excess, or whatever, the fact is that the fixture will move, and as it moves, build up speed, meaning that the load placed on the safety wire when it is suddenly stopped, will be greater. The sooner a fall is arrested, the less load is placed on the equipment. Seen less often in safety systems for equipment but more often in fall protection for personnel, is something called a "fall factor". This relates to the load placed on equipment due to the distance of the fall, with the common example being the location of anchor points. There is no reason why you can't relate fall factor theory to the safety'ing of lighting fixtures or rigging equipment, the same principle does, in essence, apply. The less distance a falling object travels, the less load it will place on the point that it is anchored to, and the equipment securing it to it's anchor. (In a common lighting example, the anchor is the lighting bar and the equipment securing it to the anchor is the safety wire).

[Ashley R]

I only ask, as currently on my fixtures, I either slip the end of the cable over the yoke (take the bolt off one end, and slip it on that way) or I loop the cable back through the end around the yoke, then place a cable tie just to ensure it doesn't come undone, the cable tie takes no load and it just there to keep the thing around the yoke of the fixture.

 

I've been doing this to stop safety's going 'missing' off of fixtures after a hire. There is no law here in Oz requiring safety's to be stamped with a SWL to my knowledge, so a lot of the time I have to just do it by eye and gut feeling as to weather a safety is up to the job, and I usually end up with 20/30kg rated safety's holding up Par Cans. Quite peeved off that I couldn't get one of my 50kg rated safety's on my Mac's (its a monster made from 4mm cable).

 

Interestingly, just looking at the Entour user manual, and it hits it 'choking' the safety cable around the truss, and then clipping the carabiner onto the safety point.... http://www.blue-room.org.uk/public/style_emoticons/default/wink.gif

[dbuckley]

[The] second and more significant effect is that if a bond is choked like in the picture above, then in arresting a fall the connector (carbine hook, carabiner, whatever) is going to be subject to a dynamic side load, which is extremely unlikely to be a mode for which it is rated and certainly one in which it will be considerably less strong and thus more likely to fail.

 

The thing on the end of the wire I didn't know the name of, but a look in the Backstage Handbook tells me its called a locking link or a quick link, like this thing:

 

http://www.snareshop.com/images/QL.png

 

The test certificate of the Selecon wire assembly can be seen here (PDF). The fixture weighs 3.3KG.

[jonathanhill]

The thing on the end of the wire I didn't know the name of, but a look in the Backstage Handbook tells me its called a locking link or a quick link, like this thing:

 

http://www.snareshop.com/images/QL.png

It is called a maillon rapide, or quick link.

 

 

 

 

[librarian28]

Chris Higgs throws a delicious spanner when, in one of his books, he suggests (if I paraphrase correctly) that an overly long safety bond could be advantageous because it may act like a bungee and take a lot of the force out of the snatch.

 

This link gives some clarification to the terms we loosely use - but it still does my head in

http://www.irata.org...fety/WLLSWL.pdf

 

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

 

If you make up a wire with a Nicopress swaged eye at both ends and take it to a test facility, you will see with your own eyes that it is always the wire that fails before the ferrule. There's a theory that the swage is twice the strength of the wire itself but, of course, that can't be demonstrated. Doubly interestingly, the wire tends to break at a point one third of its length from an end. An academic theorised that it is to do with harmonics?

[dbuckley]

While we're throwing spanners, in the case of the Selecon lantern above (and any fixture that has a safety loop point on the back) of it will behave differently to the case of a simple "loop round the yoke safety" should the yoke fail. In the simple case the lantern will fall straight down, and be stopped by the safety wire. With the Selecon as the wire becomes taught it does so at the "end" the lantern, so the lantern will then swing as it is pivoted at one end. Thus I theorise that conversion of kinetic energy from drop to swing will reduce the snap stretch on the safety.

[ramdram]

Well DB, I see what your suggesting, that energy is absorbed within the system but could you not say too that the swing increases the speed and and thus the force or load? Anyone who did Applied Math care to comment?

 

I noted too that an earlier post mentioned the wire's OD ref the rating. Ashley R remarked that the 4mm OD bonds he is familiar with are rated to 50kgs, yet the ones from Stagepoint (see earlier post) are rated thus:

 

4mm x 600mm soft loops SWL 175Kg

 

(Btw I presume you would need to connect the two loops with a suitably rated maillon rapide, or crab'?)

 

I suggest in the latter example the wire cable is a "better" quality. Perhaps AR might trundle over to his convenient wire retailer and get some info on the different types of wire and see if he could get the wire bods to make him some safeties with swaged loop (and thimbles?) in an OD to fit his kit? Perhaps the same outfit could do a batch test with certificate? Might be worth a punt.

 

I know we can in UK but the testing is extra.

[Peter Tovey]

[The] second and more significant effect is that if a bond is choked like in the picture above, then in arresting a fall the connector (carbine hook, carabiner, whatever) is going to be subject to a dynamic side load, which is extremely unlikely to be a mode for which it is rated and certainly one in which it will be considerably less strong and thus more likely to fail.

 

The thing on the end of the wire I didn't know the name of, but a look in the Backstage Handbook tells me its called a locking link or a quick link, like this thing:

 

http://www.snareshop.com/images/QL.png

 

The test certificate of the Selecon wire assembly can be seen here (PDF). The fixture weighs 3.3KG.

 

Hi David - apologies if unclear in the post you quote above, but I didn't mean to imply I was having difficulty in identifying the maillon in the picture :)

 

Have had a quick look at the test certificate from Selecon, and am surprised to report that had I printed it, it wouldn't be worth the paper it were printed on. The cert does not identify the 3 different components tested, critically it does not specify the configuration in which they were tested, it makes no allowance for the potential for dynamic loading (in a safety bond assembly!), and appears to think load is measured in kg. I am a pedant (hell, what's the point in having degrees in this stuff), but the fixture doesn't weigh 3.3kg: that's its mass.

 

Brainwave - bang on about keeping the bond as tight as possible and thus avoiding the whole dynamic loading issue. Like the analogy with PPE fall factor, had never really occurred to me before. Of course, if you're thinking about permanently rigged fixtures (based on an inspection every 6 months implying they don't move in this time) - your RA MAY not identify safetys / secondary suspensions as being a necessary safeguard...

 

Chris is absolutely right about the long bonds. Extra length means more stretch (like a bungee), and thus a greater time to arrest the fall from its initial velocity (which is higher also due to the extra length of the bond). Net effect on max deceleration thus force still relies on material properties and may increase or decrease with increasing bond length. But a longer bond will mean more swing if it ever does catch something, which will usually be adverse. Sure Mr Higgs was playing Devil's Advocate rather than giving practical advice...! Still, food for thought. Btw, as the swing is tangential to the bond, then, ignoring negligible centripetal effects, swinging does not increase the load imposed on the bond. David is correct in the case of the Selcon with the fall not being vertical - the effect of the initial swing is to increase the time it takes for the lantern to reach its lowest vertical point, thus reducing the maximum deceleration and so force.

 

Ashley - choking in itself not necessarily bad (in fact often a very useful technique) - just a few tripping-points to look out for like ensuring connectors are not incorrectly loaded, or the choking leading to non-load bearing components forming part of a load chain. If a bond with an eye one end and carabiner on the other were choked around a bar through its eye and then attached to a fixture with the carabiner, this would be perfectly acceptable practise.

 

Ramdram - this difference in ratings almost certainly not down to quality and construction of the SWR, as it doesn't have that great an effect. For varying types of SWR I see quoted MBL varying between around 9kN and 11kN for 4mm OD. The 175kg SWL is approx a 5:1 or 6:1 safety factor on this, and thus relates to use as static rigging. The same 4mm bond might well be marked up as a safety bond suitable for use of fixtures of up to 50kg mass, having de-rated the assembly to account for the potential dynamic loading knowing the length and construction of the bond and assuming a worst-case inflexible anchor point for the top of the bond.

 

Librarian - re failure one third from end - fascinating. Do you have a reference for that? Piqued my curiosity and I can't help wondering if the same would hold true for monofilament wire or if it is related to the 6-strand construction of most SWR...

 

Anyway, back to the day-job!

 

Pete

[Andrew Edwards]

Sorry guys, it was as I said Friday afternoon.

 

It's funny how your brain works as I originally put trunnion instead of yoke and then quickly changed it at the last minute. I wish I'd left it now...

 

I'm starting to wish I had put in all the little caveats about perfect systems and the obvious external forces. I also realised that I missed off the important details about the bond not being actually that long and was quite rightly pulled up.

 

My shoddy run through of the 2 equations was to prove that it wasn't [all] to do with Newton's second law which I did to be fair precede it with. I did also say about removing the slack too.

 

It's all in here *taps head* but last week I didn't quite extract and convey it very well.

 

I have a Apollo right arm. The supplied safeties are only 1.5mm. I was surprised that it was this thin. The terminations on both of them didn't have any dead sticking out of the ferrule and this couple with the diameter caused me to raise a query with the supplier. They contacted Apollo and I was duly supplied with replacements (carried by hand across the Atlantic by Apollo on the way to plasa), an apology for the terminations and a test certificate. It turns out that the WLL is 59kg and the minimum breaking point for the safeties tested was 313kg. The right arm is 13kg and then obviously the weight of any fixture you care to attach up to 18kg.

 

The rest of my 'supplied with the fixture' safeties are 3 and 4mm. The 4mm ones having a WLL of 35kg.

 

This is looks like different suppliers/manufacturers applying different safety factors depending upon the fixture. Or it could be a US vs UK thing.

 

I have the paperwork, I have an assurance from Apollo and the well known supplier. It still looks slightly scary though. Trouble is the hole through which the safety is threaded will not take a rope of greater diameter.

 

Pretty much all of my safeties have a equal or greater diameter than my (Rope Assemblies) terminated SWR I use for flying. Most of the safeties have a lower WLL however. They have all been de-rated already so by introducing a further double de-rate by my selection, I am confident that it is up to the job.