Trim Chains

[DuckJordan]

I'm going to make this easier to understand. A CM Lodestar 2 ton hoist is only capable of lifting 2 tons, after 2 tons it will not lift. There is a clutch that when it gets to the limit will keep the motor from taking chain. It will not fail at 2 tons its rated at 2 tons.

So using a 2 ton motor(in this case a CM Lodestar) you need to hang it using hardware rated at a SWL (Safe Working Load) of at least 2 tons. Since industry standard is a 7:1 ratio (although personally prefer 10:1) we would be looking for hardware with a minimum breaking point of 14 tons. This includes the shackle, the steel, and where it attaches to building steel (which should also be rated to work at 14tons on that point). Its very simple.

 

[Euphroe]

Well that explains that. Next time I rig "How To Train Your Dragon" or Lady Gaga I'll be able to understand why those shows didn't fall down the last time I rigged them. Still confused about the 3/4" shackles on the apex of the deuces but you can math that for me when I'm more advanced.

Meanwhile I'm on the phone with an automation vendor who will wake up tomorrow being owned by Tait.

Me: "What wire rope do I need to order for the 32 digitally controlled 290kg-rated acrobatic winches you're selling me?"

Vendor: "Depends. What size groove do you want us to score on the drums?"

Me: "Well I'm supposed to use rope that is 10x the SWL of the system. What is the SWL of the system you're selling me?"

Vendor: "Depends. What wire rope do you plan on using?"

Me: "I want to use the skinniest wire rope that is 10x the SWL. What is the SWL?"

Vendor: "Oh, the SWL is 1/10th the capacity of the rope you use, that's called the 'design factor'. What kind of rope do you want to use? We'll groove the drum for that."

Me: "The artists probably weigh 120lbs - 160lbs or so. Is that my SWL?"

Vendor: "In that case, we'll score the drums for 1/8" wire rope. That'll leave a little extra for safety."

Me: "Is that safe on a 290kg winch?"

Vendor: "It is if they don't hit anything."

 

[DuckJordan]

I guess I still don't get the point of this discussion, other than that Automation Vendor doesn't really understand what they are selling, at least not the way it seems from that conversation. Their winch also doesn't seem to have a fail safe built in, at least not one that will stop pulling when it reaches its calibrated limit. This brings up another point. Who is supposed to design the rig, the guy buying it or the guy selling it. To me If I were selling a winch capable of pulling 290kg I would spec minimum of 1/4" wire rope.

 

[danTt]

I think the point of discussion/question/theory, and it's something I've wondered about on an occasion or two, is that if you have a mechanical lifting device (and I'd guess you can catogirize them differently, and react differently, depending on if it's a wire rope hoist, or a chain motor, or something else entirely)

With a 2000lb winch, is the 2000lbs a MBS or a SWL? Does it fail destructively at 2000lbs (just keeps going until something snaps), does it fail safely at 2000 lbs? (Clutched like a chain motor) does it fail at 2000lbs*some design factor? Just saying a 2000lbs winch leaves this ambiguous, and the answers become ambiguous. If it's a MBS of 2000 lbs, then I would say that you need to use equipment rated for a MBS of at least 2000lbs. If it's a SWL of 2000lbs, then you need to use equipment rated for 2000lbs * the design factor in use. At least that's how I'd interpret it. You can't logically say "The winch has a MBS of 2000lbs, so everything needs to have a SWL of 2000lbs" -- The weakest point is still the winch, and when that fails it doesn't matter how much the wire rope is weighted for--except that wire rope rated for 10x whats necessary will hurt a lot more when it hits you on the head

 

[Jay Ashworth]

Euphroe's (hopefully) notional automation vendor *entirely* doesn't understand what they're selling, no.

That was the best example of circular logic I've ever seen, I think, and if a vendor pulled that on *me* selling me gear to fly people, I wouldn't obscure their name when I posted the transcript here, lemme tellya.

That said, danTt, I think the point of the latter part of this thread is that any number without a label doesn't mean anything useful to anyone, and if you can't get a label to go with the measurement, even knowing what you're talking about isn't going to help you much.

 

[LavaASU]

Well that explains that. Next time I rig "How To Train Your Dragon" or Lady Gaga I'll be able to understand why those shows didn't fall down the last time I rigged them. Still confused about the 3/4" shackles on the apex of the deuces but you can math that for me when I'm more advanced.

Meanwhile I'm on the phone with an automation vendor who will wake up tomorrow being owned by Tait.

Me: "What wire rope do I need to order for the 32 digitally controlled 290kg-rated acrobatic winches you're selling me?"

Vendor: "Depends. What size groove do you want us to score on the drums?"

Me: "Well I'm supposed to use rope that is 10x the SWL of the system. What is the SWL of the system you're selling me?"

Vendor: "Depends. What wire rope do you plan on using?"

Me: "I want to use the skinniest wire rope that is 10x the SWL. What is the SWL?"

Vendor: "Oh, the SWL is 1/10th the capacity of the rope you use, that's called the 'design factor'. What kind of rope do you want to use? We'll groove the drum for that."

Me: "The artists probably weigh 120lbs - 160lbs or so. Is that my SWL?"

Vendor: "In that case, we'll score the drums for 1/8" wire rope. That'll leave a little extra for safety."

Me: "Is that safe on a 290kg winch?"

Vendor: "It is if they don't hit anything."

Erm, so if this is an actual conversation going on for a system that will actually be built to fly people, please please please get a real rigger with experience on automation and flying people involved. This sounds like no one really knows what they're doing, and thats REALLY bad when peoples lives are at stake. People flying puts dynamic forces into play. They can be many times the load and sometimes mean you need greater than a 10:1 safety factor on the static load. Their has been two major incidents involving performer flying in the last year (both at major companies)-- one resulted in a death and the other resulted in several serious injuries. This is not a place to mess around.

 

[Euphroe]

This brings up another point. Who is supposed to design the rig, the guy buying it or the guy selling it. To me If I were selling a winch capable of pulling 290kg I would spec minimum of 1/4" wire rope.

Bingo. DuckJordan gets it.

The guy buying the winch designs the rig, but the winch vendor is under market pressure to supply what the buyer wants.

After the accident, the winch vendor blames the buyer who designed the rig with too-small wire rope, and the buyer blames the winch vendor for selling a winch with too-small grooves. If anybody asks the "standard", they will be told "design factor of ten times the SWL", but the standard does not define SWL except by circular reference to an undetermined MBS. If the winch was fitted with 2000lb MBS wire rope because somebody preferred the way it looked, the defendants will cleverly say "then the SWL was 200lbs, and since the load was less than 200lbs we complied with the standard." Never mind it's a 650lb winch.

And DuckJordan is exactly right about the solution. A winch vendor selling a 290kg winch should groove the drum for 1/4" wire rope minimum and tell the client to work with that. Even then, ropes of that size vary from 5400lbs to 8500lbs.

danTt:

Winch components are engineered for various percentages of the rated load. The rigging designer does not need to know the failure load of the gearbox secondary shaft or the woodruff key -- that's the winch vendor's turf. The winch doesn't have a "minimum breaking strength". It has a rated load, generallly specced at its max speed. A 290 kg winch is about 650lbs at top speed, and maybe 1200lbs at lower speed. Beyond that it stalls.

The wire rope is a critical component, it should be strong enough to stall the winch. The winch is not supposed to pull the rope apart if the load snags -- the rope is not a fuse link, it is a critical component. So generally the SWL is the same as the winch capacity. All components downstream of the winch should have an MBS 10x the winch capacity. If they have an MBS of 5x the winch capacity, you're operating at a design factor of 5. That might be okay, but remember the winch ultimate capacity could be twice the rated capacity.

 

[danTt]

Euphroe, let's approach this a different way. Your concern at the moment seems to be that if something is hung from a hoist, and catches on something it flys out, the wire rope should not fail before the winch does. I can understand this, but then I start wondering--why? Do you really want whatever the object is caught on to be subjected to 2000lbs of force? Might this not be more dangerous than having something fail quicker?

Similarly, and to play devils advocate a little--All we are talking about at this point has been the rigging materials. If you are flying a broadway flat from a winch with a rating of 2000lbs, would you suggest that the flat needs to be constructed to withstand 2000lbs of force? If a wooden flat catches as it flys out, and is subjected to 2000lbs of force, the scenery is going to fall apart. Is this an area of safety that people should be concerned with? This becomes important to think about with motorized fly systems especially. I'd suggest some sort of load cell monitoring on hoists installed in these spaces to detect if the load changes dramatically while moving, and warn/stop under such circumstances.

 

[LavaASU]

Euphroe, let's approach this a different way. Your concern at the moment seems to be that if something is hung from a hoist, and catches on something it flys out, the wire rope should not fail before the winch does. I can understand this, but then I start wondering--why? Do you really want whatever the object is caught on to be subjected to 2000lbs of force? Might this not be more dangerous than having something fail quicker?

Similarly, and to play devils advocate a little--All we are talking about at this point has been the rigging materials. If you are flying a broadway flat from a winch with a rating of 2000lbs, would you suggest that the flat needs to be constructed to withstand 2000lbs of force? If a wooden flat catches as it flys out, and is subjected to 2000lbs of force, the scenery is going to fall apart. Is this an area of safety that people should be concerned with? This becomes important to think about with motorized fly systems especially. I'd suggest some sort of load cell monitoring on hoists installed in these spaces to detect if the load changes dramatically while moving, and warn/stop under such circumstances.

Also if you are talking about people there comes a point (as far as force) where if they were to hit something the rope breaking would be the better option... someone might survive the fall, they will not survive being torn into pieces. That said, in a performer flying system, this should not be a problem (it should not be possible for them to get caught!).

 

[cmckeeman]

Well that explains that. Next time I rig "How To Train Your Dragon" or Lady Gaga I'll be able to understand why those shows didn't fall down the last time I rigged them. Still confused about the 3/4" shackles on the apex of the deuces but you can math that for me when I'm more advanced.

The reason for the size shackles is not about SWL, but about fitting the eyes into the bell of the shackle. Crosby rates 1/2 in shackles for 2 tons but we use 5/8 for 1 tons. the other reason is that a 3/4'' shackle won't be able to rotate through the eye of the wire rope and potentially become cross loaded which is why i personally like using them everywhere except the free shackle...

 

[Euphroe]

Euphroe, let's approach this a different way. Your concern at the moment seems to be that if something is hung from a hoist, and catches on something it flys out, the wire rope should not fail before the winch does. I can understand this, but then I start wondering--why?

Because it's all relative, but it's my job to make sure the rope does not break. The flat might disintegrate without hitting anything, the girl might fall off the cloud swing because she has the flu etc., but the rope is never supposed to break. Nobody should spec a winch system where the wire rope becomes the weak link because the design factor was too low.

That said, in a performer flying system, this should not be a problem (it should not be possible for them to get caught!).

No, it is common. You have to design for it. You design for the worst-case foreseeable loading condition, which is that the load/rope is stopped against something and the winch is still pulling.

Erm, so if this is an actual conversation going on for a system that will actually be built to fly people, please please please get a real rigger with experience on automation and flying people involved. This sounds like no one really knows what they're doing, . . .

No, it's me characterizing what goes on at the highest levels of the business, riggers with vast aerial experience working with the highest-end vendors. It's not that they don't know what they're doing. It's that the door is open for them to hedge the numbers because there's no clear standard. People at this level are not infallible, and not always as responsible as they should be. The winch buyer wants to cut the margins with a different rope, and the winch vendor goes along with it. The buyer tells himself it must be okay or the vendor wouldn't have agreed. The vendor tells himself that the buyer must know what he's doing and anyway it's his responsibility.

Call them on it and they will stonewall because there is no unambiguous standard that says the rope must be 10x the winch rated capacity, especially after deducting for the end attachments. And I've VERY often seen end attachments with a book efficiency of 60%. Whether they actually operate that low is an open question.

 

[soundman]

No, it's me characterizing what goes on at the highest levels of the business, riggers with vast aerial experience working with the highest-end vendors. It's not that they don't know what they're doing. It's that the door is open for them to hedge the numbers because there's no clear standard. People at this level are not infallible, and not always as responsible as they should be. The winch buyer wants to cut the margins with a different rope, and the winch vendor goes along with it. The buyer tells himself it must be okay or the vendor wouldn't have agreed. The vendor tells himself that the buyer must know what he's doing and anyway it's his responsibility.

Call them on it and they will stonewall because there is no unambiguous standard that says the rope must be 10x the winch rated capacity, especially after deducting for the end attachments. And I've VERY often seen end attachments with a book efficiency of 60%. Whether they actually operate that low is an open question.

PlASA is working on it. BSR E1.43 - 201x, Entertainment Technology -- Live Performer Flying Systems is open for review until June 3rd.

4.8.4
Strength design factors
Lifting media shall be sized to meet the following design factors:
4.8.4.1
Flexible lifting media
Flexible lifting media (e.g., rope, chain, band, webbing) shall be designed with a minimum design factor of 10X WLL, 6X characteristic load and 3X peak load.

Peak load is defined as The maximum force applied to the performer flying system resulting from abnormal conditions, or irregular operation (e.g., effects of emergency stops, uncontrolled stops, drive electronics or power failure, stalling of the actuation equipment, extreme environmental conditions)

If you are worried about a load getting caught and the rope breaking then as I read the standard you peak load is the maximum amount of force the winch can exert at any speed. 3X to meet 4.8.4.1 and see where that leaves you.

 

[BillConnerFASTC]

I'm discussinng multi-purpose installed hoist systems and not a system designed for one show where knowing who will use and how it will be used has an impact on design choices.

If I have a motorized hoist system designed to lift 700 pounds and use a design factor of 10, then everything in the load path should be designed to hold 7000 pounds. That's hold, not lift.

PS Sorry, I missed there was a page 4 and this is a response to a much earlier post. Its the problem of spending a weekend in the woods with Scouts and not having cell service. Hope if interested you all can connect this.

 

[Euphroe]

PlASA is working on it. BSR E1.43 - 201x, Entertainment Technology -- Live Performer Flying Systems is open for review until June 3rd.

4.8.4
Strength design factors
Lifting media shall be sized to meet the following design factors:
4.8.4.1
Flexible lifting media
Flexible lifting media (e.g., rope, chain, band, webbing) shall be designed with a minimum design factor of 10X WLL, 6X characteristic load and 3X peak load.

Peak load is defined as The maximum force applied to the performer flying system resulting from abnormal conditions, or irregular operation (e.g., effects of emergency stops, uncontrolled stops, drive electronics or power failure, stalling of the actuation equipment, extreme environmental conditions)

If you are worried about a load getting caught and the rope breaking then as I read the standard you peak load is the maximum amount of force the winch can exert at any speed. 3X to meet 4.8.4.1 and see where that leaves you.

PLASA defines WLL by reference to MBS, and MBS by reference to WLL. The formula is circular until you define one or the other. PLASA does not say that WLL = winch rated capacity.

"Characteristic load"? "Peak load"? More variables.

Here's what happens in the real world:

Client orders winches rated at 650lbs at max speed, grooved for 1/4" wire rope. Vendor sells the entire automation system without asking any further questions about the lifting media, because nobody makes it his responsibility and the client is always right.

Client typically installs 1/4" 19x7 rotation-resistant at 5400lbs. Then installs swivels on the 19x7, contrary to the Wire Rope User's Manual and wire rope manufacturer's warnings/advisories. The swivels reduce the breaking strength of that wire rope by about 40%, and cause birdcaging.

So is it now 3240lb wire rope? Would you take a 20% deduct for wire rope clips used as directed? But not take a 40% deduct for swivels used against warnings?

At any rate, you now have a 5400lb - 3240lb wire rope on a 650lb winch. If the WLL = rated capacity, you do not have a 10:1. The winch maxes out at 1200lbs. If the payload hits an obstruction, you might or might not have a 3:1 depending on whether you account for the swivel in your math. Since you are not supposed to be using the swivel anyway (per the wire rope industry), you bury your head in the sand and call it a 5400lb wire rope.

That is literally how the pros do it.

Oh and the recommended D/d for 19x7 is 34. Are they using 8.25" sheaves? (Ignore it, because the swivel will cause a birdcage long before the bending causes broken wires).

There is a 1/4" 8500lb wire rope that is 100% efficient with a swivel and approved for human lifts, recommended for D/d of 20. It costs about twice as much, lasts at least 4 times as long, and doesn't birdcage. I know two shows that use it, to their complete satisfaction, and past shows also. I've used it. Why don't you see it everywhere?

* Neither the client nor vendor are responsible for treating the winch/rope combination as a single engineered system. There are OEM tires for a Hyundai, OEM chain for a hoist, but no OEM wire rope for a human-lift winch. Anthing goes. Install 7x19 from Home Depot and nobody will notice or care.

* Wire rope is treated like an expendable, interchangeable product like gaff tape, rather than a special component like hoist chain.

*PLASA won't address the Wire Rope User's Manual or manufacturer's warnings about swivels, because that would be like giving a monopoly to the one company that produces a wire rope for this application.

* Human lifts aren't important enough yet. One show flys people on 8500lb wire rope and changes the rope 4x per year. Another show from the same company flies people on 5400lb wire rope that has to be changed monthly. Both use the same model winches. Why? No reason, they just do.

*Politics/pride. Riggers do not like being told they could be doing a better job, or that their math is off. Offer them an improvement and instead of being grateful, they feel accused and think about how it will look if they switch.

If I were a winch vendor, I would spec an OEM rope for my winches based on the output and without regard to what the client intends, 8 months before opening, to do with that winch. If the client wants to deviate after receiving the winch that's his responsibility.

 

[soundman]

PLASA defines WLL by reference to MBS, and MBS by reference to WLL. The formula is circular until you define one or the other. PLASA does not say that WLL = winch rated capacity.

"Characteristic load"? "Peak load"? More variables.

I don't think the standard refers to MBS.

2.40 working load limit (WLL):
The maximum weight as defined by the Flying System Designer that a User is allowed to apply to a lifting medium in the performer flying system.

That is literally how the pros do it.

That doesn't mean its the best way or even right. As more companies are getting into the winch building game I think having a defined process for specing hardware is going to become very important to clear up confusion. Perhaps one day it will by similar to hanging a chain motor where there is standard hardware sizing, but its not there yet.