BillConnerFASTC
Well-Known Member
Me too. Could she pull that up just a little more?
Trim Chains
- Thread starterkicknargel
- Start date
When I said that to the rigger, she slapped me.
AlexDonkle
Active Member
As a curiosity, one thing I've yet to fully understand is why trim chains are even used in permanent install. What advantage do they have over batten terminations using pipe clamps with turnbuckles? Trim chain seems faster for temporary work, but I'm not entirely sure why it's common in permanent install as well.
BillConnerFASTC
Well-Known Member
I don't like the way turnbuckles project outward in a slack line condition, and catch things. But I don't disagree with your premise.
Euphroe
Member
Maybe people accept it because OSHA may not apply to certain lifts in audience view. Like a magician's "assistant" or a prop levitating on piano wire. Which is very different from the liftlines of a CW set.
The more interesting question is this: is the SWL of a counterweight set always based on a fully-loaded arbor, no matter the "intended" load? Assume an 8' arbor is maxed, and the load is concentrated on one liftline; then 1/4" 7000lb wire rope makes sense, with matching hardware.
How does this relate to the "intended" load? If two sets have 8' arbors, but one is "intended" for a cyc can you use smaller trim hardware than if the set is "intended" for an LX?
Why should a 7-line set with an 8' arbor require the same hardware as a 4-line set with an 8' arbor?
Now, how does this relate to a motorized set? Should the size of the wire rope and hardware be based on the winch capacity (as with hoist chain and hardware), or based on the "intended" load? If I only "intend" to fly 50lbs, can I install 500lb wire rope on a 1000lb winch?
We seem to size counterweight components based on the max possible load.
Theaters systems remain in use for sometimes upwards of 50 or 70 years. You cannot predict how someone will use those systems, nor do you want to limit how they can use those systems. I've seen sets intended for cycs use to hang hard-covered scenery. I've seen shows where the audience is seated on stage and just about anything can become fair game for deviating from normal practices and uses. Designs for systems like rigging, lighting, audio, and video should not be looked at as designs for systems that will only be in place for 5-10 years. They are infrastructure for supporting the growing and changing needs of those venues over decades to come. While electronic equipment may be replaced more frequently, the other backbones of theaters (power and data distribution, rigging sets and points, lighting positions) will remain much longer and see far more different uses over the years.
As an example, at our design/install shop, we specifically pull spare network cables in our data conduits knowing if they won't use them right away, there's an excellent chance they'll be used in the future. We take into special consideration that installing those wires and conduits with in the first place so that the venue has room to grow is much more cost effective than later on if they have to hire an electrician to install those conduits and wires. Especially if installation would mean coring through concrete walls and floors, or trenching through concrete slabs. If we limit our design to the bare necessities on the first go 'round, when the venue does inevitably want to upgrade in 10-15 years, they can expect to pay a disproportionately higher cost than if they had put that infrastructure in place on Day One.
Euphroe
Member
You're saying the design factor should be based on the potential force in the system.
Since an 8' arbor could be fully loaded at some future date, the components should be sized for the capacity of the arbor, not the "intended" load.
Say its a temporary installation and I'm using a 1T hoist, but only I only "intend" to fly 800lbs. Can I use an 880lb quick link to hang the 1T hoist to the beam?
Or is it like the counterweight set where the components should be based on the potential force in the system (hoist capacity) and not the "intended load" (load weight)?
If a balanced CW set strikes an adjacent pipe, the force is only what the rigger pulls.
But if a hoisted load strikes an obstruction, the force is the capacity of the hoist and the quick link will break.
So do we work from the "intended load", or from the potential force in the system? Because the two are not interchangeable.
Since an 8' arbor could be fully loaded at some future date, the components should be sized for the capacity of the arbor, not the "intended" load.
Say its a temporary installation and I'm using a 1T hoist, but only I only "intend" to fly 800lbs. Can I use an 880lb quick link to hang the 1T hoist to the beam?
Or is it like the counterweight set where the components should be based on the potential force in the system (hoist capacity) and not the "intended load" (load weight)?
If a balanced CW set strikes an adjacent pipe, the force is only what the rigger pulls.
But if a hoisted load strikes an obstruction, the force is the capacity of the hoist and the quick link will break.
So do we work from the "intended load", or from the potential force in the system? Because the two are not interchangeable.
I'd advocate for potential force, especially in extended use or permanent installations. Intent today and intent next month or next year can be drastically different. Temporary situations are different though in that you have more control over intent. That rigging used to hang a chandelier for a show on Monday is unlikely to be repurposed on Tuesday for hanging a line array. That's more of a "design for today's application" scenario than the "design for the future possibilities" scenario.
As for the the quick link, no you can't use it unless you find one that's rated for overhead lifting, and I only know one manufacturer in the world who makes those.
As for the the quick link, no you can't use it unless you find one that's rated for overhead lifting, and I only know one manufacturer in the world who makes those.
DuckJordan
Touring IATSE Member
On top of that Euphroe, most companies specify what kind of loading you can do using a single point on the system. The single point load on our battens with, I think, 6' arbors is 700lbs, thats as much as you can put on a single point. if spread out evenly accross the 6 lift lines the rating goes up to arbor max which is 2500lbs per line set.
BillConnerFASTC
Well-Known Member
I don't know what an "intended load" is. We design overhead lifting systems with safe working loads in mind, and have signage stating what that is. Loading on one line of a set, on a batten, and a combination of lines up to all of them in a set. This is in turn used to design the structural framing. And this applies to general purpose linesets on a stage that is not built for just one production. Electrics and shell sets have slightly different criteria, and we post that to.
Euphroe
Member
DuckJordan: are those double arbors? 2500lbs would be 34lbs per inch. Either way, it seems trim chains are sized based on arbor capacity, not on intended load.
MNicolai and everybody else: Design factor is supposedly the minimum breaking strength over the "safe working load". Current standards seem to conflate SWL with "intended load".
But what is the "safe working load"? It is whatever somebody decides can be flown on the hardware or the system. If the venue installs a 50,000lb winch with no wire rope on it, but tells the crew the "safe working load" is 100lbs, then it is a 100lb winch with a design factor of 500.
But . . . then the crew decides to install 1000lb minimum breaking strength wire rope on the winch, based on the 100lb SWL of the system, and tells themselves the wire rope has a design factor of 10.
So we end up with a 1000lb wire rope on a 50,000lb winch. Have we followed the rules?
Euphroe
Member
Hi Bill,
"intended load":
1926.1431(g)(3)Rigging hardware (including wire rope, shackles, rings, master links, and other rigging hardware) and hooks must be capable of supporting, without failure, at least five times the maximum intended load applied or transmitted to that component. Where rotation resistant rope is used, the slings must be capable of supporting without failure at least ten times the maximum intended load. (italics added)
Contrast the above with this:
1926.1414(b)(2)
Wire rope must be designed to have, in relation to the equipment's rated
capacity, a sufficient minimum breaking force and design factor so that
compliance with the applicable inspection provisions in § 1926.1413 will be
an effective means of preventing sudden rope failure.
Design factor is MBS/SWL. But what is the SWL? It is MBS/design factor.
It is circular. The only non-relative term is MBS.
We need another non-relative term. It should be the potential force in the system. NOT the "intended load". Because if you put a 1000lb wire rope on a 1000lb winch you are asking for trouble, even if the "intended load" is 100lbs.
The reason I piggybacked this on the "trim chain" discussion is that we seem to intuitively accept that CW rigging components should be based on the potential load in the system, that is the arbor capacity.
What happens if 1000lb counterweight lineset snags on the way out? The added force is the 40lbs or whatever the flyman applies. It's nothing, almost no risk of catastrophic failure.
But what happens if a 1000lb winch snags when lifting a 100lb load? The "intended load" becomes irrelevant. What matters is the potential force in the system.
BillConnerFASTC
Well-Known Member
DuckJordan
Touring IATSE Member
When did I say we ever max out our arbor with weight limit. We have never hit weight limit other than the arbors size limit. That motors load rating doesn't mean anything when its hung using 1000lb cable. Its wll is now 100lbs using a 10:1 ratio, that limit is not to be exceeded which is why we use safety factor. We don't load to breaking limit in case a snag or other issue occurs. I'm not sure what if any point you're trying to make is coming across.
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Sent from my XT1060 using Tapatalk
Euphroe
Member
Sent from my XT1060 using Tapatalk[/QUOTE
No, winch capacity means everything. In case of DuckJordan's "snag or other issue", the tension in the line becomes the winch output, and an attempt to "de-rate" the winch by signage will prove only that winches can't read signs.
The TD writes "SWL 200 lbs" on the wall next to a 2000lb winch. The show flies a performer at 10:1 design ratio, so the crew installs a 2000lb 1/8" wire rope on a 2000lb rated-capacity winch.
Acceptable? or not acceptable? The wire rope has a 10:1 ratio to the SWL signage and to the performer's weight with an allowance for dynamic load.
But it's a 1:1 ratio to the winch capacity, and that's a really dangerous game to play. In event of "snag or other issue" the winch will feed its 2000lbs tension into the line, the rope breaks and the load falls, because physics beats signage, every time.
Winch wire rope size should be based on concrete numbers, like this:
MBS / (winch rated capacity) = 10
5000lb winchline/ 500lb winch = 10
(Also, most variable speed winches increase torque as they slow. A 2000lbs winch may be pushing close to 4000lbs at low speed. This is part of what a 10:1 based on winch capacity is supposed to cover, and why a 2000lb winch gets a 20,000lb+ wire rope.)
No, winch capacity means everything. In case of DuckJordan's "snag or other issue", the tension in the line becomes the winch output, and an attempt to "de-rate" the winch by signage will prove only that winches can't read signs.
The TD writes "SWL 200 lbs" on the wall next to a 2000lb winch. The show flies a performer at 10:1 design ratio, so the crew installs a 2000lb 1/8" wire rope on a 2000lb rated-capacity winch.
Acceptable? or not acceptable? The wire rope has a 10:1 ratio to the SWL signage and to the performer's weight with an allowance for dynamic load.
But it's a 1:1 ratio to the winch capacity, and that's a really dangerous game to play. In event of "snag or other issue" the winch will feed its 2000lbs tension into the line, the rope breaks and the load falls, because physics beats signage, every time.
Winch wire rope size should be based on concrete numbers, like this:
MBS / (winch rated capacity) = 10
5000lb winchline/ 500lb winch = 10
(Also, most variable speed winches increase torque as they slow. A 2000lbs winch may be pushing close to 4000lbs at low speed. This is part of what a 10:1 based on winch capacity is supposed to cover, and why a 2000lb winch gets a 20,000lb+ wire rope.)
DuckJordan
Touring IATSE Member
So according to you we should never fly anything with less than half inch cables? Education is what matters if you have to always design for idiots there will always be better idiots.
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BillConnerFASTC
Well-Known Member
1. Winches are used for pulling and hoists for lifting. I thought this was about hoists and lifting.
2. A motorized system designed for a swl of 200 pounds for overhead lifting should not be capable of lifting much more than 200 pounds. The hoisting capacity of the power train is usually not and should not be 8 or 10 times the swl.
2. A motorized system designed for a swl of 200 pounds for overhead lifting should not be capable of lifting much more than 200 pounds. The hoisting capacity of the power train is usually not and should not be 8 or 10 times the swl.
Euphroe
Member
Bill, you use "SWL" twice in the above paragraph, as though SWL is separate from the rated capacity of the winch/hoist.
Where does this "SWL" come from if not the winch capacity?
From the "intended load"?
SWL = MBS/(design factor) My design factor is 10. My winch capacity is 700lbs. My intended load is 200lbs.
What is my SWL?
