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councilman24

Not parachute but sewing.

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I'm well familiar with sacrificial bar tacks on parachute bridles and fall arrestors. Attached is a photo of what I think are supposed to be sacrificial stitch patterns on a tree stand harness. This is the tree strap and has the webbing folded and stitched. No other reason than shock absorption. While not 5 cord I think the stitch pattern may be as strong as the webbing. In addition the structural joints are not much if any different. Anybody even seen 4 point w stitch patterns as sacrificial joints inteneded to fail? Seems like a poor design. Of course a chinese pos.
I'm old for my age.
Terry Urban
D-8631
FAA DPRE

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Yeah... and if I can do the math in my head, if 5 cord at 40 each, 320 lbs for each "tooth on the zipper" if the threads are supposed to break sequentially. Of course, fewer pounds if the thread is a lighter weight or polyesther. I doubt it would accomplish the shock reduction function it seems it is intended to do. I agree it would probably just stop the falling 'tree stander guy'.

However, to be fair, I am only an "armchair doubter". And, even though the cross stitch is strong if the straps are pulled lengthwise, it isn't strong when loaded = thread by thread. That's why they have confluence wraps on military risers. It would have to be tested with a couple of hundred pounds and enough length to get the breaking effect to work. Maybe it has been tested and works correctly? 200 lbs moving downward fast may be enough? Only a test would reveal it's design success or failure. I'm not impressed by the fuzzy /loose webbing. When the stitches break- if they ever do, the webbing will be ripped a bit too.

(I'd kinda also wonder what the anchor would be to take such a shock. It better be good.)

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I've wondered before...

Are these fold-things supposed to save the operator?

Or are they only to detect that the one-time use has occurred?

If the threads are very weak, it might not save anything but just act as a detector.

(Just asking and thinking out loud. I'm not making any claims.)

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Thread appears to be the same as the load bearing patterns. I assume they are meant to be shock absorbers. I'm trained in high angle rescue and since we use static lines (minimal stretch) we use shock absorbers made of sacrificial bartacks. But these are loaded in peal. Butler uses the same thing on his HX round series bridles and Free Flite on Preserve V's. But these aren't loaded in peal, they're loaded in shear.
I'm old for my age.
Terry Urban
D-8631
FAA DPRE

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Hi Terry,

Quote

But these aren't loaded in peal, they're loaded in shear.



I am of the opinion that only testing will determine this.

If loaded in shear, then I doubt that they can be a 'shock attenuator.' Shear usually goes all at once; yes, I know a thread or two will break, but then everything lets go.

Peal would IMO be a 'shock attenuator;' a few stitches at a time.

Sew up a sample and test & see what you get; then let us know.

Jerry Baumchen

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riggerrob

On the subject of shock attenuators: I saw them on seat-belts on a recently-manufactured van. They were regular seat-belt webbing with the sacrificial stitching loaded in peel.



They are called 'load limiters' and I believe they have been on all cars since about 2008, along with 'pretensioners' which take up slack in the belt when a crash is detected. The load limiter can take many different forms, from the stitching you saw to devices that involve the deformation of metal in various ways. Some designs have the pretensioner and load limiter incorporated into the same device to save space, weight, cost.

Vehicle safety has come a long way, and is a good reason to own a newer rather than older car. If we could get people to accept a racing type harness to restrain their upper body instead of just a single strap across, many more lives would be spared along with far fewer injuries.

In my previous life I was a mechanical design engineer for Honda.
People are sick and tired of being told that ordinary and decent people are fed up in this country with being sick and tired. I’m certainly not, and I’m sick and tired of being told that I am

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sundevil777


Vehicle safety has come a long way, and is a good reason to own a newer rather than older car. If we could get people to accept a racing type harness to restrain their upper body instead of just a single strap across, many more lives would be spared along with far fewer injuries.

In my previous life I was a mechanical design engineer for Honda.



In my previous life I was a race car fabricator. With every good idea there are a few bad side effects. Trouble with kids putting in 5 point racing harnesses is that they roll the car, the roof collapses and it snaps their neck because their upper body is immobilized against sideways motion.

-Michael

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