Light fixture safety cables
- Thread starterDagger
- Start date
Not thinking wrong, that's why fall distance is an important part of the shock load equation. But so is the force of the stop. A more gradual stop equals less shock load. The question wasn't, "will there be a shock load on a safety cable," but, "how is shock load calculated?"
Gobokat
Active Member
The simple answer to your questions is to use the simple F=ma formula, but it gets complicated with a shock absorbing lanyards in fall arrest systems because they're meant to change the "a" part of the equation by the way they're sewn. If you want to play around with different deceleration time scenarios try the calculator on this page -
Calculate the force of a fall while using the fall protection force calculator
Use our free fall protection force calculator to determine the force generated on the body when using fall protection.
wilmes.co
And for what it's worth, there isn't enough elasticity in a wire rope type safety cable to count as any appreciable deceleration so your falling lighting fixture is pretty much straight F=ma calculation.
correction: *stopping* after falling.
12 pounds falling is under a (basically) constant 32 ft/s/s acceleration (well, until you reach terminal velocity) and thus the falling force is constant. F = m * g. The mass and acceleration aren't changing while it's falling.
The difference is that the safety cable is only going to deflect so much and that proportion (and proportion of time - the math is easier with time than distance) is different over the longer distance. I'll leave this calculation as an exercise for the reader but suffice it to say falling farther causes a higher velocity when under constant acceleration and thus the acceleration back to not moving* is going to be quite different.
(I did do rough terminal velocity math on the aforementioned 5/8 nut but it seems by my quick math it won't reach terminal velocity until it has fallen over 130 feet, more than the 10/50 split suggested)
*ok, it's probably going to bounce around for a while. Don't try this at home (well, unless this testing is the kind of thing you do for a living -- in that case, please do and post youtube videos with all the math too!)
Sheesh. Again, just addressing the question that was asked, which was, "How is shock load calculated?" Weight, fall distance and stopping distance all factor into it.
But I'm just some schmuck on the internet; if you really want to know, Jay Glerum, Delbert Hall, and Harry Donovan all have excellent books that will explain it better than I could.
But I'm just some schmuck on the internet; if you really want to know, Jay Glerum, Delbert Hall, and Harry Donovan all have excellent books that will explain it better than I could.
Another example. Please help me with the calculations
I have seen ona gig where a motor chain fall out of a bag while it was being raised .
The motor was hung invert 100ft up with gac in a basket configuration. When the chain was being raised approximately 80ft up in the air the chain fell out of the bag. ( wrong size chainbag ).
What is the shockload?
I have seen ona gig where a motor chain fall out of a bag while it was being raised .
The motor was hung invert 100ft up with gac in a basket configuration. When the chain was being raised approximately 80ft up in the air the chain fell out of the bag. ( wrong size chainbag ).
What is the shockload?
Crisp image
Well-Known Member
Or a Maillon Rapide. A great alternative and very secure.
EDIT: Corrected spelling
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I had to look that up (and not just because you spelt it wrong).
Maillon Rapide
Here in the US, we'd call that a quick-link, and endlessly debate its appropriateness.
.
Correct
Incorrect. You are combining stopping distance and fall distance.
Fall distance = how far the fixture falls before the safety engages and it begins to slow down
Stopping distance = How far the cable 'stretches' before stopping the fixture. The longer that distance is, the more time it will take.
Force = Mass * Acceleration
Acceleration = Change in Velocity / Time
In our case here, Acceleration = slowing down the fixture from free fall
The longer the time it takes to slow down, the lower the acceleration will be. Lower Acceleration = lower force.
Correct.
Shockload force = ?
LOAD = 150lbs (CM motor 125' chain ~150lb)
Free fall distance = 80'
Stopping distance= ??? ( it fell 80'and came to a stop- so hoe to find value for that??)
Also back to LOAD: use only the weight that fell? Which is 80'of chain , idk maybe ~40 lbs or use the entire weight of rhe motor?
And where is the shock load applied to ? I am thinking it is applied to the shackle on top ( which is making the connection to gacflex)
@egilson1 @TheaterEd @RonHebbard @RonaldBeal @porkchop @What Rigger?
@JAC
@bobgaggle
@BillConnerFASTC
Etc
Crisp image
Well-Known Member
No matter what you call them they have a rating which is MBS not WLL or SWL. As long as they are done up correctly they are good but must be used as designed.I had to look that up (and not just because you spelt it wrong).
Maillon Rapide
Here in the US, we'd call that a quick-link, and endlessly debate its appropriateness.
.
I have safety cables in the PAC that I work at with them. The beauty of them is that it takes multiple operations to open them and there is no chance of roll out.
If I had a choice of these and some of the clips on other cables I would choose theses everytime.
Regards
Geoff
BillConnerFASTC
Well-Known Member
BillConnerFASTC
Well-Known Member
Good thought but I'd find it a little impractical to inspect and see at distance - like mousing can be seen - and if it bounced or the safety cable loop is rolled/rotated the link would be upside down.
I'm in a minority on the safety cable on lights - I don't loop through yoke but cinch around the eye on the fixture (or around the pipe) and clip to itself (or to the eye). The yoke to pipe doesn't protect from what I think is the most common failure point - the yoke bolt. And IIRC the mouse requires all accessories - color frames, top hats, etc. -to also be saftied. I've only ever seen a barn door fall. YMMV
1/16"gac has a breaking strength of 480 pounds so 60 SWL if the swages and hook is rated for 100%? Put two on movers.
Crisp image
Well-Known Member
Since we're veering down the path to the tightening of Quick-Links / Maillion Rapides; an area amateur group, a group where many of the performers were teachers, from kindergarten through colleges to universities and (in their minds) beyond with the bulk of their builders, sound and lighting members being their students; many afflicted with SEVERE Queen / King Of THE Booth syndrome:
The "teachers' were of three minds:
- Find the BIGGEST wrench you can muster and strip the threads of every shackle, turnbuckle, and quick-link on the premises.
- Leave them finger tight.
- And - or; they're only students, phuqu'em; let's go to the bar and drink ourselves silly!!! (then drive home [that'll teach 'em!] )
Consequently I'd periodically visit and light a production; often when voluntold by my wife (but that's a whole different story)
Hence many students grew to believe that finger tight, or looser, fittings actually magically tightened themselves over time given enough time between productions. Obviously, what other explanation could make sense: Little Johnny lit a show and knows he left things finger tight, only to return next semester and find he needed to clamber back down and climb back up, an un-tethered wrench (or two [who knows what sizes may be required]) in one hand as he scampers up their rolling set of stairs.
Who'd ever have thought some guy from Stratford would drop by and be IDIOT enough to actually bring his own 8" Crescent (with a 'silly' lanyard attached) wrench with him to actually touch his wrench, lightly, to each and every overhead fitting that would only need to be loosened a few weeks or months, later. Toodleoo!
Ron Hebbard
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BillConnerFASTC
Well-Known Member
We are trying to protect against 1 in hundreds of thousands.
