Break cord / tape grades

[mathrick 2]

I'm looking for ways to create a self-detaching camera mount that will be stable and secure in normal conditions, yet will reliably be ripped off should it ever entangle with any part of the deployment system. One of the ideas I'm considering is based on the use of break cords, as they're designed to do exactly that. It might be a total dud, which I won't know without some testing, however I've run into an unexpected difficulty when trying to get some for playing with.

The only type I can find anywhere is the 80lbs MIL-T-5661, as sold and used for BASE static line attachments. But going by the ballpark figure of the PC pull force of 75lbs (there's a huge variability in reported and calculated values for different PCs and freefall times, so I might just as well pick Bill's guideline value of 75lbs), it won't be strong enough to break the cord, and even if I were to go by the much larger figures of 160+lbs at terminal speeds, it would definitely lead to significant delays on slow deployments (ie. cutaway and reserve extraction).

The value that would seem more reasonable is in the 30-50lbs range. But it's not just that I have difficulty locating retailers, I can't even find any references to any grade of break cord other than the 80lbs ever existing. Am I just blind and can't search? Are they called something else than break cord/tape when not sold by BASE stores? Are there any other similar materials with close tolerances for their breaking strength that could be used?

"Skydivers are highly emotional people. They get all excited about their magical black box full of mysterious life saving forces."

[JerryBaumchen 1,048]

Hi mathrick,

Quote

the 80lbs MIL-T-5661

You should understand that almost every cording, tape, webbing utilizes a minimum breaking strength.

I am very sure that your 80 lb cord will go much higher than the minimum 80 lbs as rated.

There might be some products out there with a maximum breaking strength, but I have yet to find one.

Jerry Baumchen

[pchapman 261]

Just speculation:

One step would be to get a hold of a Poynter's manual.

Break cord is the Type I, 1/4" version of that MIL-T5661. Most others are higher strength. One could also look at thread & cord, there's a whole range of sizes. An example is 5 cord at 31 or 40 lbs minimum depending on whether it is Nylon or Cotton milspec style.

But what's actually available easily, I have no idea.

All the numbers are minimum breaking strengths. How much tolerance there is, constrained by a requirement to be under a certain weight per length, who knows. Probably not too much but one would want to test.

Some skydiving stores do have Light Super Tack, at 50 lbs instead of the usual 80 lb.

Then there's always the 'small nylon bolts' that people talk about for ring sights etc. One can get some at Home Depot (in the US/Can). But I don't know their strengths. With a ring sight the idea might be that even if a caught line doesn't snap it off, manual grabbing the sight will provide plenty of leverage to snap it.

I imagine a tricky part with breakaway mounts is the issue of forces applied in different directions. If the mount is taller, one can get issues of leverage. A straight up pull might stress the break cord directly, while a bump on the front or back can add a lot of stress from leverage -- whether that is a line catching it at an angle, or bumping into another jumper on a bad group exit. That makes it harder to know what strength is correct to help in an emergency but not lose your expensive camera too easily.

The graphic below shows the issue: If a mount sitting on a helmet with a single break cord at the bottom breaks after 50 lbs of direct pull force, then if the geometry gives a 3 to 1 leverage ratio, a 17 lb bump will snap the camera off. A lower, squatter mount would help, especially if one were doing a low profile side mount rather than a taller gopro style thing up top. (Multiple smaller ties at the corners might help too, but that gets into other issues.)

So does one go for a breakable mount, or just go for a mount with a manual release?

[inline detachable-camera-mount-leverage.jpg ]

[councilman24 36]

Jerry I haven't looked at.the spec but.since.the.80lb cord is used to close military static line rigs I assume it has both a minimum and max breaking spec.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[pchapman 261]

Who knows, maybe they do expect a maximum in service.
But if the constraints according to Poynters are
a) min 80 lbs breaking strength,
b) max .011 oz/linear yard, and
c) cotton material,
then maybe there isn't a lot of room to play in terms of strength, unless better weaving technology and improved cotton genetics have changed things since the spec came out. Plus someone might not sell to a milspec if their version costs a bunch more.

In any case, since we're not talking about volume production, then the OP can test whatever roll he's using.

[mathrick 2]

@Jerry: the MIL-T-5661 is specifically specced to have both minimum and maximum breaking strength at 80lbs with very close tolerances. It's used for static line applications, where you need to ensure that the attachment point will hold and lift your D-bag, but will break once under load.

@peter: yeah, those are the considerations. I wanted a to-spec cord precisely because you never really know otherwise. It's also a problem with the nylon bolts, because almost none of the ones I've seen in retail are rated for specific forces and what you get is essentially a crapshot, with much higher shear strengths than people imagine to begin with, even for rated components. But yeah, nylon screws are another angle I'm approaching it from.

The mount is meant to be a side-mount for a small sony cam on my Rev2. The idea I'm sitting with at the moment is a pair of slightly concave (exaggerated in the drawings) circular pads, bound together in the middle with the break cord (red line).

[inline camera-front.jpg][inline camera-side.jpg]

The hope is that such a configuration would create mostly uniform leverage regardless of the direction of the pull. The lighter coloured gray bumps in side view are for locking together the pads without twisting and sliding.

"Skydivers are highly emotional people. They get all excited about their magical black box full of mysterious life saving forces."

[JerryBaumchen 1,048]

Hi Terry,

Quote

minimum and max breaking spec

When I am using the maximum breaking strength I mean a value that it will never go above.

When I was still working as an engineer, we had some breakaway bolts on some very specific equipment. The head was built/machined with a smaller section where it would break at a ~ predetermined torque.

That's as close as I have ever come to a 'max strength' item.

Jerry Baumchen

PS) I did a test last year with Type 8 webbing. The stitching broke at 5,400 lbs. Type 8 is rated at 3,600 lbs min.

[councilman24 36]

The Preserve V round by FreeFlite specs a very light weight break tape. I don't remember the spec and I'm not home and on my phone but look up the manual and you'll find it. The stuff is so light that you have.to be careful tying it to keep.from breaking it. Two.turns might work. I'm sure there must be break tapes specked in between. We used light weight wire ties as staticline PC assist connectors. They sometimes spec strength.

Also I use the velcro like heavy duty connector that uses plastic mushroom shaped posts interlocking to hold led lights on altimeters. It used to be sold to hold camera's, may still be. Check Paragear or ask in photo forum.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[gowlerk 1,911]

Quote

the MIL-T-5661 is specifically specced to have both minimum and maximum breaking strength at 80lbs with very close tolerances.

I know it's used in this fashion, but in looking for specs I can find no mention of any such maximum strength rating. At first I thought Jerry was wrong as well, but although it is commonly called "break tape", I don't see any maximum spec at all. I think it only got that reputation because it was used that way. It's actually spec'd as a reinforcing tape.


Spec sheet can be found here.
http://everyspec.com/MIL-SPECS/MIL-SPECS-MIL-T/MIL-T-5661E_40576/

Always remember the brave children who died defending your right to bear arms. Freedom is not free.

[mathrick 2]

Right, the things you learn after you make a public post on the 'tubes. It also has the property of getting stronger with higher humidity, which is rather less than optimal.

"Skydivers are highly emotional people. They get all excited about their magical black box full of mysterious life saving forces."

[JerryBaumchen 1,048]

Hi mathrick,

Quote

the things you learn

At the end of the day, this is what counts.

There is absolutely nothing wrong with being incorrect. When you find out that you were incorrect and learn from that, then everyone learns along with you.

That's the important part.

Jerry Baumchen

PS) I've been wrong on here more than once.

[councilman24 36]

Recognize that the spec you linked to is way out of date, dated in the early 90's. The Feds abandoned that spec in 1998 along with all the mil-specs for parachute fabrics and hardware. The first FAA rigger handbook published in 2005 erroneously still called them mil specs and many catalogs and web sites still refer to those spec numbers as mil-specs. But PIA took them over after the Feds abandoned them. Otherwise every maker would have their own specs and every contract would have had to specify the individual characteristics. PIA is on the 4th revision of that particular spec since 1998, the last one issued in 2015. It is not available for free, PIA sells them to the industry on their web site. I don't know if it now lists a max breaking strength. It may very well not but I think the next size up was a min breaking strength of 120 so it would be pretty safe to say it wouldnt be higher than that.

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[Alexg3265 0]

the common rubber bands we use are 40lbs break strength on average... i use one on my gopro just wrapped around the screw just incase a riser knocks it out of its little slide thing... just saying..... Another thing to look at is try looking at threads... nylon thick threads. maybe 3 cord could work depending how many attachment points,,... I think you should be looking for threads rather than a tape or ribbon per se.... actually maybe wrapping paper ribbon might break around where you need... zip ties? get a fish scale and just start pulling and breaking shit... youll find something

I was that kid jumping out if his tree house with a bed sheet. My dad wouldn't let me use the ladder to try the roof...

[councilman24 36]

councilman24

We used light weight wire ties as staticline PC assist connectors. They sometimes spec strength.

For instance, and not the ones we used.
http://www.nelcoproducts.com/4-miniature-cable-ties

and

http://tapecasecom.blob.core.windows.net/product/download/70070940443.pdf

I'm old for my age.
Terry Urban
D-8631
FAA DPRE

[hackish 8]

Honestly I think you're going about the problem in the wrong way. What's needed is a low snag mount made from a plastic that breaks given a certain amount of force. With a cast plastic design you have some control and repeatability.

With a fabric type attachment you are going to have issues keeping the camera snug and the material from being worn, torn or otherwise violated when installed on one of 100 types of helmets available.

-Michael

[Hellis 0]

I had a day in the workshop a few years ago to figure out how break cord works.
It is very concistent at 80 lbs or whatever the load is supposed to be depending on the setup. (Loop = 160 lbs)

However, inspect the cord for defects, a smal error in the pattern means a big loss in strenght.

Also with your setup you shoud think about the movements will "eat" the materail.

This is by no means sscientific, but this is some videos of how break cord handles load and different static line setups.
The excelfile is probably in swedish and most likely hard to read even for a swede. It was never intended to be read by others, it was just my sum up of the tests.

http://217.209.138.12/Videoklipp/2012/Fallskärm/Dragtest/

[mathrick 2]

hackish

Honestly I think you're going about the problem in the wrong way. What's needed is a low snag mount made from a plastic that breaks given a certain amount of force. With a cast plastic design you have some control and repeatability.

With a fabric type attachment you are going to have issues keeping the camera snug and the material from being worn, torn or otherwise violated when installed on one of 100 types of helmets available.

Aye, doing it with plastic latches is another idea I've considered since. I'm not targetting any helmets other than my own, so I will make it extremely specific to the Rev2 attachment point and shell shape. It won't be cast plastic (I'm not doing big runs, just my own individual fabrication), but I have access to 3D printing which will hopefully be repeatable enough.

@Andreas, Terry: thanks, will give it a look.

"Skydivers are highly emotional people. They get all excited about their magical black box full of mysterious life saving forces."

[hackish 8]

3d printing would do exactly what you need. I have seen them before. As long as you can put up with the colour selection...

-Michael

[gb1 2]

Have you thought about manually releasing the mount? Some kind of a quick release operated with your hand may be worth looking at instead of hoping cords will break when they should.

[mathrick 2]

Indeed, my earlier attempts at a design had that, and the helmet has a built-in cutaway too. But manual release systems require composure, mental power, time, and not having a snapped neck to activate; all precious commodities when you're busy entangling with your last remaining canopy. So I'm trying to make it release on its own, since I cannot completely prevent snags, given the shape of the camera and the helmet.

"Skydivers are highly emotional people. They get all excited about their magical black box full of mysterious life saving forces."

[gb1 2]

I understand about requiring time, and the snapped neck. How about a fairing to get rid of all corners?

[Hellis 0]

mathrick

Indeed, my earlier attempts at a design had that, and the helmet has a built-in cutaway too. But manual release systems require composure, mental power, time, and not having a snapped neck to activate; all precious commodities when you're busy entangling with your last remaining canopy. So I'm trying to make it release on its own, since I cannot completely prevent snags, given the shape of the camera and the helmet.

Sorry mate, I call bullshit.

Unless you have a camera they usually have in a news station studio, I'm not buying it.

I bet it will take you less than 20 minutes to fix the real problem with polymorph instead of spening hours building a "auto cutaway".
https://youtu.be/IhVuc6RNyaw

[mathrick 2]

Not possible because of how Rev2 is shaped and operates and the fact it's a side mount. The camera also has to be offset forward a bit from the mounting plate to avoid having a large portion of the FOV obscured by the visor, which presents extra snag and pinch points. A fairing is possible to prevent lines flying back to front (as seen by the helmet) from catching onto things, but front to back, as well as bottom to top (think PC catching onto your wrist and releasing just as your hand is close to your face) will always have a chance of getting jammed between the visor and the camera. Making the camera sit completely flush with the shell is impossible due to the fact the whole front hinges up and down. If you take a look at Rev2's product page, you can see the mounting plate -- it's the round washer with three mounting holes. It doesn't rotate together with the visor, so it always stays in the same orientation.

"Skydivers are highly emotional people. They get all excited about their magical black box full of mysterious life saving forces."

[mathrick 2]

Hellis

Sorry mate, I call bullshit.

Unless you have a camera they usually have in a news station studio, I'm not buying it.

I bet it will take you less than 20 minutes to fix the real problem with polymorph instead of spening hours building a "auto cutaway".

Have you actually looked at the helmet? It's not possible if I use the mounting washer plate; all the moving parts are right there and there's simply no way to make it sit flush.

I could make a permanent fairing if I mounted it on the audible cover sitting below. In that case, I'd have to add 2-3cm of spacing between the shell and the camera to reduce the amount of visor caught in the FOV to acceptable levels, but that opens me to riser strikes and I don't know enough to know if I should be worried about those. It'd also make the whole thing fixed and not removable ever, and make angle adjustments difficult to impossible. All things you can live with, I suppose; people have jumped with cameras 3x the size side-mounted, but I just don't know that it's obviously a better solution than dicking around with a breakaway mount.

"Skydivers are highly emotional people. They get all excited about their magical black box full of mysterious life saving forces."

[Hellis 0]

Well....
Then maybe you have the wrong helmet for the type of jumping you want to do?

And why would you not be able to adjust the angle if you attach it to the audible pocket?
Or what makes it adjustable on the other position higher up? You said the round thing there by the visor does not rotate (?), so how does that make it more adjustable than not being able to rotat a few inches lower?


The Contour has (had) a similar shape and had a rotatable mount. It did stick out from the helmet, but this gap can be filled with polymorph.