Chest strap replacement concerns

[andyhughes 0]

I saw an interesting chest strap replacement on the weekend, would be interested in hearing comments/opinions on the mod…

Container is a Javelin odyssey with chest rings and the strap was made from a single layer of type 17. The chest strap was threaded through the chest ring with 3 inches of turn-back. The attachment had a buffer (type 12 I believe). My initial concern was with the stitch pattern used to secure the turn-back to the chest strap: three vertical bar-tacks were used rather than the usual stitch patterns used on harnesses (although I couldn’t identify the thread type).

Riggers, would you have repacked this rig? Would you be happy to make such a mod?

Jumpers, would you be happy with this mod and trust your life to it?

[riggerrob 643]

I have seen dozens of Javelins with bar-tacked chest straps and have never heard of any of them failing.

On a similar note: circa 1990, Rigging Innovations certified (full range of TSO drop tests, etc.) the Flexon with a harness held together by bar-tacks. To this day, there are still hundreds of airworthy Flexons with bar-tacked harnesses.
The only reason R.I. did not make bar-tacks a long-term production standard is that sewing machines wore out too quickly sewing through all those layers of webbing.

In conclusion, you can sew a harness with a few stitches of 5-cord (old school) or you can sew it with hundreds of stitches of E-thread (Javelin chest strap, the top end of most main risers, etc.).
If anything, bar-tacks spread the load better.

[andyhughes 0]

Thanks very much for the info.

I guess the main difference between those methods would be that strength of a bartack junction is much more directional than the "old school" method? But for things like the above chest strap and skyhook attachments etc that isn't really an issue.

How would you estimate the strength of the bartacked junction? When the force is 90degrees to the stitch direction would it be aprox "number of stitches X thread strength"? How about when the stitch direction is the same as the load? (assume no point loading i.e. force and stitch direction are on the same plane)

[RiggerLee 61]

There's a much simpiler issue here. It's an alteration to the harness. Unless this modifacation has been approved by the manufactorer or FSDO and signed off by a master rigger then the TSO is dead. You have a very nice climbing harness, Sex swing, wall ornament, etc.
Wait, climbing harnesses have some kind of cert. Sorry it's just a sex swing.

Lee

Lee
lee@velocitysportswear.com
www.velocitysportswear.com

[andyhughes 0]

Yes, the mod had been signed off by a master rigger.

Any suggestions regarding the strength of a junction (see above post)?

[riggerrob 643]

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Yes, the mod had been signed off by a master rigger.

Any suggestions regarding the strength of a junction (see above post)?

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

Odd that he would only use a single layer of Type 17.

Granted, 2500 pounds worth of Type 17 is probably strong enough, but the production standard is two layers of Type 17 (total 5,000 pound tensile strength). The primary reason for two layers is to "bulk out" the light weight friction adapter (buckle), to prevent it from slipping when not loaded (i.e. while sitting in airplane or sitting in freefall).
With the massive overlap (on double layer chest straps) the stitch pattern becomes irrelevant.

IOW there are so many hundreds of stitches (E Thread at 8.5 pound tensile strength) that the bar tacks become a formality.

[mark 107]

Quote

Granted, 2500 pounds worth of Type 17 is probably strong enough, but the production standard is two layers of Type 17 (total 5,000 pound tensile strength). The primary reason for two layers is to "bulk out" the light weight friction adapter (buckle), to prevent it from slipping when not loaded (i.e. while sitting in airplane or sitting in freefall).
With the massive overlap (on double layer chest straps) the stitch pattern becomes irrelevant.

IOW there are so many hundreds of stitches (E Thread at 8.5 pound tensile strength) that the bar tacks become a formality.

I agree that a single layer of Type 17 is probably strong enough -- the weak point is the friction adapter -- but I disagree about the bartacks becoming a formality on a Javelin chest strap.

There is only one layer of Type 17 around the chest ring, and it ends just beyond the 3-bartack pattern, which also secures the other end of the webbing. The only load-bearing overlap is at the bartacks.

Mark

[andyhughes 0]

Ok, so lets break out the calculator here a second… for this chest strap mod there are 3 bartacks, that looked to be around 42 stitches each… so this would give 126 stitches. So if type-e thread was used we would have a theoretical (shear) strength of 126 * 8.5lb = 1071 lb. Take off 15% for thread wear (from sewing), which gives us a max strength of 910 lbs. That is less than half of the strength of the single layer type-17 used to make the strap: a load that I would say is fairly achievable? Hence the alarm bells…

If this mod was done using FF, then we would get a junction strength similar to that of the webbing. But I’m not sure how much trust I would put in a harness system with a theoretical max load of 2000lb on the chest strap… I’d certainly prefer 2 layers.

[riggerrob 643]

Note that the fashion for chest straps - for many years - was a single layer of Type 8 webbing (4,000 pounds).
Type 8 chest straps only broke when buckles had sharp edges. Also remember that Type 8 chest straps were usually sewn to main lift webs with a 1.5 inch by 1.5 inch WMW stitch pattern of 5 cord. Type 8 chest straps only fell out of fashion when sit-flying fell into fashion, which led to problems with loose chest straps coming loose from buckles.

Nowadays, a double layer (total tensile strength of 5000 pounds) of Type 17 (2500 pounds) is fashionable for chest straps. Please remember that the double layer was not chosen for strength, rather two layers are needed to "bulk out" the buckle, to prevent it from slipping when not loaded.

[mark 107]

Quote

Ok, so lets break out the calculator here a second...

Type 17 webbing rated for 2500 pounds. Stitch pattern good for about 900 pounds. 1" quick fit adapter rated for 500 pounds. What will fail first?

RiggerRob is right: the reason for two layers of Type 17 is to reduce slippage, not to add strength.

Mark

[pchapman 279]

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Type 17 webbing rated for 2500 pounds. Stitch pattern good for about 900 pounds. 1" quick fit adapter rated for 500 pounds. What will fail first?

I'd like to know what the real strength of chest strap adapters are, whether the one for the 1" webbing, or the traditional wide-webbing MS70101 design.

Although they are somewhat wimpy pieces of metal, they are much maligned as everyone keeps quoting the 500 lbs number.

"Rated strength" does not tie in directly with ACTUAL STRENGTH.

It's all a bit mysterious and I've never gotten good answers (or found them in DZ.com searches).

RATED STRENGTHS have different safety factors depending on the standards for that type of item. For example:

-- Webbing:
Poynters suggests that a rated webbing strength is the minimum at which the webbing would break. If there is not more to the story, the manufacturer would make sure that for any variation in material and manufacture , their webbing will break at that strength or somewhat higher. There is no extra safety margin built in, above the quoted strength.

(I'm discussing the new webbing alone. It's another matter how it is used, given stitch pattern strengths or UV and mechanical wear)

-- Rapide quick links:
The loads listed on them are Safe Working Loads, with a 5 times factor to the Breaking Load. The Proof Load that they must withstand without permanent deformation is half of the Breaking Load (or therefore 2.5 times the quoted Safe Working Load)

-- Parachute hardware:
(At least for the US style milspec hardware)
Poynters #1 is vague. It says that hardware is proof loaded, and that there is a 100% safety factor. So I guess but don't know that if hardware is quoted as "500 lbs" then it is supposed to hold 500 without permanent deformation, but also to survive 1000 without actually breaking (if the load is applied in the intended manner).

If this is correct, then it suggests that in the quoted scenario, then the stitch pattern of 900 lbs max may actually be fairly well matched to chest strap hardware that could well go to 1000 lbs before failing.

[TonyJ 0]

If this mod was done using FF, then we would get a junction strength similar to that of the webbing. But I’m not sure how much trust I would put in a harness system with a theoretical max load of 2000lb on the chest strap… I’d certainly prefer 2 layers.

[andyhughes 0]

good point mark. the 500lb strength of the adaptor will probably go first.

Regarding my estimation of 910lb for the stitching, i have 2 questions:
1. Would you say that was realistic. I am not sure what extent the webbing would be damaged by the super close stitch pattern.
2. Would you say that the strength of the bartack would be the same across the direction of stitching as the same direction? How much would it differ?