Has anyone done post-repair proof loading?

[FrogNog 1]

There is some question about how safe it is to unstitch and restitch harnesses, in terms of final strength.

I know there is theory (with some testing behind it) that will explain the strength of webbing before and after it has received a reasonably-well-known number of potentially fiber-cutting punctures. And stitch theory (based on lots of testing) explains how much stitching it takes to bond webbing in certain ways for certain strength.

But is there any actual testing of webbing that has been punctured a multiple of the appropriate number of times (e.g. instead of 50 punctures for a stitch pattern, give the webbing 500 punctures) followed with proof or destruction strength testing?

I am uncomfortable that the state of the art could be riggers saying "I think it's OK to repair something once, but not twice." (No offense meant to the guy who actually said that; there's nothing wrong with your opinion, I'm just saying I'd like some practical science.)

-=-=-=-=-
Pull.

[councilman24 36]

Bill Booth may chime in here. Dan Wilcox, who worked for RWS at one time, presented a lot of damage strength testing at the PIA symposium in 1991 or 93. I've often referred to photos from his test samples on what kind of damage results in what level of strength decrease. I don't remember if that particular presentation included repairs but I believe it did. I know it stressed the danger of nicking the webbing with a razor or knife when taking harnesses apart. I know that Bill has done a lot of tensile testing in relation to all sorts of problems. I'd expect, but don't know, that the French on their static load testing table have probably done similar kinds of testing.

But, aside from a couple of design problems, (like the "DEATHSTAR" TRAC II) harnesses don't come apart. That is until the two failures last year of student harnesses. I know those are being investigated and no real conclusions had been reached last I knew.

You should get some of the old timers at your DZ to haul out some of the junk we used to jump. I have a ratty old PROTOTYPE Super Swooper Tandem (SST) that I jumped for a couple of years that I would even think of putting on now.

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

[JerryBaumchen 1,082]

This 'problem/rumor' has been around for a long time. Back in the 60's I wrote an article for PARACHUTIST and part of the article was about removing stitching/resizing the harness (old military B-4/B-12 types) and I got hammered for even thinking about removing and redoing some stitching.
Interestingly, just two days ago I tested a 3" long 3-point stitch using #5 cord. Failure occured at 5,760 lbs; way above what I ever would have expected.
If I get some time after the first of the year I might just run a series of tests; but remember these are time consuming. But then again, the old engineer in me likes to 'know' things.

[mjosparky 3]

Jerry.

What type of webbing were you using and how many stitches per inch?

Sparky

My idea of a fair fight is clubbing baby seals

[riggerrob 563]

Much of this testing was done before you were born and since then riggers have just followed "common practice" as written in Poynter's or old Air Force manuals.
Every once in a while, some one like Jerry Baumchen gets bored, repeats tests and reaches the same conclusion: harnesses are built way over strength.
As long as you do not accidentally cut webbing fibres, you can probably resew a harness a half dozen times. The difference in strength between a piece of webbing that has been sewn once, versus three times, is negligible. Good needles don't damage webbing.
However, most harness manufacturers limit repairs to sewing a piece of webbing a maximum of three times.

[JerryBaumchen 1,082]

Sparky,

Something to understand: When one sets a sewing machine to stitch X stitches per inch it is important to understand that it will not do that absolutely. The number of stitches per inch will vary somewhat; a negligible amount in the real world but it will vary.

I wanted to determine the strength of the sewn joint and not of the webbing itself. A little R&D for myself and my curiousity; again, that old engineer in me.

I have my machine set for 6 stitches per inch and I used Type 7 webbing.

Hope this answers your question.

[mjosparky 3]

Thanks Jerry. Some day I would like to spend some time following you around. The possibility of learning things would be great.
Sparky

My idea of a fair fight is clubbing baby seals

[JerryBaumchen 1,082]

As my grandmother used to say 'Be careful of what you ask for, you just might get it.'

[riggermick 6]

Quote

Much of this testing was done before you were born and since then riggers have just followed "common practice" as written in Poynter's or old Air Force manuals.
Every once in a while, some one like Jerry Baumchen gets bored, repeats tests and reaches the same conclusion: harnesses are built way over strength.
As long as you do not accidentally cut webbing fibres, you can probably resew a harness a half dozen times. The difference in strength between a piece of webbing that has been sewn once, versus three times, is negligible. Good needles don't damage webbing.
However, most harness manufacturers limit repairs to sewing a piece of webbing a maximum of three times.

Couldn't have said it better Rob!

Mick.

[deadwood 0]

PIA 2003
Storm Dunker (with Rigging Innovations) showed the results of load testing comparing re-sewn seams on Type VII webbing, 5-cord, 3" 4 Pt stitchs at 5 SPI.

There was little to no difference on the first re-sew, (actually showed a small increase in strength)
2nd resew showed about an 8% drop. 3rd and 4th resews showed no further strength loss.

It was an interesting presentation that reviewed alot of other variables on seam strength. line SPI, patterns, webbing treatments, confluence wraps, thread tension, etc.

He who hesitates shall inherit the earth.

Deadwood
Skydive New Mexico Motorcycle Club, Touring Division