Braided steel shock cord?

kuririn said:

Yeah, I'm gonna backtrack on the picture wire being O.K. I just remembered that black powder residue is corrosive to bare metal. That's why you see gun enthusiasts seemingly obsessively cleaning their firearms: it's to keep the internal parts from pitting and corroding.
Also picture wire will break after repeated bending, like a paper clip that is bent back and forth. Use a high tensile clear coated steel cable instead.

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Theoretically that corrosive will cause surface flaws and affect life cycle of component from machine design knowledge as a surface defect slight crack propagation and the surface finish itself. And your other steel wire will have a higher life cycle before failure but it will still fatigue fail. I've done textbook integral problems in material science for some applications such as heavy aircraft spars they say several million cycles. Other applications like this rocketry shock cord stuff I just won't trust it. It bends and could twist too. You may have not seen torsional failures. I have seen a inch diameter steel rod A-36 structural just torsion fail by twisting in a lab machine. It took very few rotations to happen under harsh loading. It looked like a candy cane and appearance changes then it starts necking then a loud kink noise as it fails. Though I'm just an undergrad mech. But yes they still show the paper clip fatigue failure in class to engineer students.

Not trying to bash anybody's feelings. Try to use better fireproofing materials, higher temp epoxies,
multiple mount points, or shielding tactics around non metal cords nylon, Kelvar, etc. okay with metal D links or even bolts. But a shock cord... I pause.

rharshberger said:

And you have a lot of experience with them....?

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Torsional tested steel rods for mechanics lab. Seeing a 1" diameter A-36 structural steel solid round bar go candy cane mode then fail suddenly while twisting was enough. It'll take a torsional deflection angle, up to a certain point elastically,but repeated twisting is what gets it. Under permanent deformation in twisting failure mode the visual indication is increasing bands of darker material appearance form increasingly closer together. It'll give you cold chills when you see it. Here's a solid steel rod and now it's a freaking candy candy then it snaps. Many people yap of metal fatigue and never know torsional fatigues exist. I've heard chutes can twist. If you guys are using it as Y yoke then maybe fine. But if it's a cord that could twist I'd go dork screaming like a little scared moron. In a shock cord it's not fixed in a machine rest at ends in a known controlled lab test environment, it's just out there and can bend or twist whichever way. Then you think well... Explicitives.. Harder to predict a failure. So my dumb logic here is avoid it by not picking it. Automotive axles with soft lower grade China steel also failed in torsional modes under normal use. S7 Tool steel. Just my 2 cents of rambling.

Andrew_ASC said:

Torsional tested steel rods for mechanics lab. Seeing a 1" diameter A-36 structural steel solid round bar go candy cane mode then fail suddenly while twisting was enough. It'll take a torsional deflection angle, up to a certain point elastically,but repeated twisting is what gets it. Under permanent deformation in twisting failure mode the visual indication is increasing bands of darker material appearance form increasingly closer together. It'll give you cold chills when you see it. Here's a solid steel rod and now it's a freaking candy candy then it snaps. Many people yap of metal fatigue and never know torsional fatigues exist. I've heard chutes can twist. If you guys are using it as Y yoke then maybe fine. But if it's a cord that could twist I'd go dork screaming like a little scared moron. In a shock cord it's not fixed in a machine rest at ends in a known controlled lab test environment, it's just out there and can bend or twist whichever way. Then you think well... Explicitives.. Harder to predict a failure. So my dumb logic here is avoid it by not picking it. Automotive axles with soft lower grade China steel also failed in torsional modes under normal use. S7 Tool steel. Just my 2 cents of rambling.

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So in other words NO. You have limited rocketry experience and keep trying to apply academic knowledge to a hobby level project, eventually when you have actually tried these systems then you will be able to offer experienced input. Stainless steel wire braid or twisted wire cables are fairly resistant to the heat and corrosion for our uses of them. Most of the failures of these types of cables for me have been A) the cable eyelet or crimp fails, especially the premade leader type, B) the point they are anchored to fails, for both of thses reasons I have moved to using systems tha allow easy replacement of the recovery shock cord/cable, usually kevlar or eyebolt and kevlar combo. Eventually when I have time and spare cash to buy a proper crimping tool I will revisit stainless steel shock cord anchors, they have evidently been functional and part NCR kits for many years (which is something I learned from this thread).

Andrew_ASC said:

Theoretically that corrosive will cause surface flaws and affect life cycle of component from machine design knowledge as a surface defect slight crack propagation and the surface finish itself. And your other steel wire will have a higher life cycle before failure but it will still fatigue fail. I've done textbook integral problems in material science for some applications such as heavy aircraft spars they say several million cycles. Other applications like this rocketry shock cord stuff I just won't trust it. It bends and could twist too. You may have not seen torsional failures. I have seen a inch diameter steel rod A-36 structural just torsion fail by twisting in a lab machine. It took very few rotations to happen under harsh loading. It looked like a candy cane and appearance changes then it starts necking then a loud kink noise as it fails. Though I'm just an undergrad mech. But yes they still show the paper clip fatigue failure in class to engineer students.

Not trying to bash anybody's feelings. Try to use better fireproofing materials, higher temp epoxies,
multiple mount points, or shielding tactics around non metal cords nylon, Kelvar, etc. okay with metal D links or even bolts. But a shock cord... I pause.

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I shouldn't have to say this, but 1" A-36 rod is far, far different from braided or laid steel wire rope. Wire rope is meant to bend, especially in the 7x7 or 7x19 lays (1x7 and 1x19 are a lot stiffer). So if it bends back and forth, it's doing its job. If the wire rope is stainless, it's a lot more corrosion resistant than A-36, and 316 stainless is even better than 304. There's a possibility that wire rope will have fatigue problems at a hard spot where it's glued into a bulkhead, but Kevlar will as well.

I've seen an A-36 rod break the FIRST TIME it was pulled! Of course if you pull something past its ultimate strength it will break. For fatigue loading, they showed you breaking the unit in a few cycles because that takes a lot less lab time than trying to get 1E6 or 1E8 cycles. If you put 5000 pounds on a 1000-lb WLL wire rope, it'll break too. That doesn't mean the wire rope isn't suitable to retain rocket parts, as long as the loading stays well under the WLL.

Your posts remind me of a joke: A German car engineer is walking around a factory in Detroit with an American engineer. When the American points out a novel thing they've done, the German sniffs and says, "That's all well and good in practice, but it will never work in theory!" There is nothing more terrifying in an engineering office than an intern or recent graduate who thinks they know everything because they know the theory.

boatgeek said:

There is nothing more terrifying in an engineering office than an intern or recent graduate who thinks they know everything because they know the theory.

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Having worked in manufacturing, in an engineering department for, oh, along time (I used to draw with a pencil!). I've seen my fair share of these. And mechanical engineers who couldn't fix a bicycle, or even knew what a the 6 and 32 meant on a 6-32 screw. I've seen mech designers (engs) also design overly complicated pieces, because they could (easy to do in CAD). One older company I worked for, approached a local university to help solve a problem / find a newer & better solution. They came up with one. Pretty high tech. Our cost was in the $20K range to produce. This was for a device we typically sold for $2K.. Yeah..

just because you're smart, doesn't mean you're smart...

Andrew_ASC said:

Torsional tested steel rods for mechanics lab. Seeing a 1" diameter A-36 structural steel solid round bar go candy cane mode then fail suddenly while twisting was enough. It'll take a torsional deflection angle, up to a certain point elastically,but repeated twisting is what gets it. Under permanent deformation in twisting failure mode the visual indication is increasing bands of darker material appearance form increasingly closer together. It'll give you cold chills when you see it. Here's a solid steel rod and now it's a freaking candy candy then it snaps. Many people yap of metal fatigue and never know torsional fatigues exist. I've heard chutes can twist. If you guys are using it as Y yoke then maybe fine. But if it's a cord that could twist I'd go dork screaming like a little scared moron. In a shock cord it's not fixed in a machine rest at ends in a known controlled lab test environment, it's just out there and can bend or twist whichever way. Then you think well... Explicitives.. Harder to predict a failure. So my dumb logic here is avoid it by not picking it. Automotive axles with soft lower grade China steel also failed in torsional modes under normal use. S7 Tool steel. Just my 2 cents of rambling.

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Making a mess of 1" ~70,000PSI steel? That's cute...



As so many have stated, just because one type of steel in one shape fails when subjected to a load designed to make it fail does not mean all steels are going to fail when subjected to a vaguely similar situation. May steel grades are flexible and have extremely high tensile strengths for their mass, they are temperature and chemical resistant and easy to coat with plastics to further improve that resistance.

dr wogz said:

Having worked in manufacturing, in an engineering department for, oh, along time (I used to draw with a pencil!). I've seen my fair share of these. And mechanical engineers who couldn't fix a bicycle, or even knew what a the 6 and 32 meant on a 6-32 screw.

just because you're smart, doesn't mean you're smart...

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And I've met corporate CEO's of multimillion dollar foreign German automakers too arrogant to ask the most experienced line technical operator what the problem with the assembly line was. He face to face asked me the unknowing fresh minted intern mechanical student what the problem was. I had no car assembly experience. I stated calmly talk to Ruth he's worked the line for seven years, and he told me the PLC logic control issue starts at the door gate. Would you believe eventually a engineering director got fired by other engineering directors for a bad standard operating proceedure manual and that alcohol wipes that pinched pennies destroyed a transparent plastic form of glass like material's clarity? A contractor has only specified polymer wipes. What did that practically do? It destroyed the laser beam tolerance through the surface which causes laser beam to deflect off center then read a false hole depth reading on part alignment of a sub component of a German automobile rail frame member in a assembly line. The machines were giving part insertion robot faults to operators and mechanics and the entire line would randomly crap out. That CEO could have fired me on the spot for sounding like an A-hole. He said he never thought to ask a line worker once in his entire life. I could've written twenty pages on what was broken that I observed but they didn't want that. They were arrogant. They had statistics, we had to get data for those statistics and eight pages of robot faults a day wasn't helping. They lost millions of dollars. And they were missing the problems we saw on the line. They eventually put directors on the line asking us more questions and seeing how f---Ed a car factory floor really is. You had maps marked left and right backwards and the Germans called it correct meanwhile the left and right rails were going to wrong places in a car factory. They designed a car and SUV and couldn't admit their left from right on a map was wrong. Nor did they ever give workers radios. People had to run fifteen minutes across a plant to get a mechanic who would radio for a software programmer, an electrician, or eventually an engineer team. And logistics never had a clear idea of when a parts bin ran out. And they never had spare robot parts but they have every part to make three different cars next door and a rail/truck depot. They never saw airline maintainers or how FAA does it where every spare part exists and stuff really got fixed not just slap a reset button then run off before an error pops up again. They lost eight out of eleven hours on robot faults per shift. You ever got dreams of robot faults in German? The workers laughed and said that's normal about week four. They zip ties safety sensors and people would crawl into the pit.

8,800 spot welds per vehicle and the only answer you could get was add more robots. Only VW of Germany knew those part dimensions. And they couldn't make a tool head bolt on a robot adapter rail to do more spot welds at once??? They said the following word. Impossible. Oh they would buy a robot factory tool. But they weren't going to manufacture it to do their tasks quicker. Here's one never knew welding slag caused resistance faults that wasn't in a textbook. I asked how they figure out how many spot welds and they said DOT wrecks the cars so they interate until it passes.

My worst day there was waiting for the engineering directors to finish a meeting, they were mad I wasn't at a assigned position. I waited for them to finish talking. I said a MiG welder robot missed welds on a structural beam, you know only structural beam. Nobody else had the bAlls to say anything. They closed the plant for four hours and found eleven vehicles missing welds in places where welds should have been. Everyone was sprinting with sharpies. Maybe I saved someone that day. Those words it dropped the weld nut plate.

I can take apart a Volvo and talk to mechanics with diagnosis. I think six is hole size and thirty two is thread per inch. I've machined flywheels on lathe and ran a CNC mill, but I try to ask people that know better or more in different ways. I don't care their rank or job they always have some experience better than mine.
But I'll ask this, what's the theory for a braided cable twisting failure because I've never learned that?

Andrew_ASC said:

But I'll ask this, what's the theory for a braided cable twisting failure because I've never learned that?

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It doesn't matter, because if you're clever you put a swivel on it and then you don't have to worry about it twisting and failing.

Corrosion or not I've found attaching steel cable a bigger PITA than it's worth. I'd just assume attach some kevlar or tubular nylon to a traditional attachment point. Won't snag, way more flexible and far easier to pack.

Thanks.

The way I see it, Ya either use it, or not....

dr wogz said:

Having worked in manufacturing, in an engineering department for, oh, along time (I used to draw with a pencil!). I've seen my fair share of these. And mechanical engineers who couldn't fix a bicycle, or even knew what a the 6 and 32 meant on a 6-32 screw. I've seen mech designers (engs) also design overly complicated pieces, because they could (easy to do in CAD). One older company I worked for, approached a local university to help solve a problem / find a newer & better solution. They came up with one. Pretty high tech. Our cost was in the $20K range to produce. This was for a device we typically sold for $2K.. Yeah..

just because you're smart, doesn't mean you're smart...

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For a good engineer, it only takes a couple of times when the guys in coveralls tell you to come down and weld it together yourself if you think it can be done. I think I made it to 3 or 4...

Andrew_ASC said:

...
But I'll ask this, what's the theory for a braided cable twisting failure because I've never learned that?

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You're asking the wrong question. The theory for what happens when you twist cable is awful. Sort of like how it's horrendously difficult to make a theoretical model of a curling stone sliding down the ice with sweepers in front. What you need is practice. Twist a cable one way, and it makes a loop. Twist it the other way and the lay opens up into a birdcage shape. If you twist it too much, you'll permanently wreck the lay of the strands and the cable is toast. For straight up tension tests, it's pretty much the same thing. There's a lot going on mechanically in a cable as it comes under load and breaks. However, they are very reliable in how strong they are if they're cared for. If you do choose to tension test one, make it a short length and stay well away because there's going to be cable strands whipping everywhere.