Closing loop physics

[hackish 8]

From a strictly technical point of view Airtec says that threading through the 3 holes of their washer reduces the amount of pressure on the knot to 30% when the loop is under tension. Since the breaking strength of their closing loop is listed as in excess of 450lbs does this mean that the knot does not reduce the strength of the system to 80% as they typically would in a straight tensile test?

I also notice the photo posted of the vigil washer posted by erdnarob http://www.dropzone.com/cgi-bin/forum/gforum.cgi?do=post_attachment;postatt_id=101145;. Assuming I am correct about the ultimate strength above is correct then how would one consider the strength of the vigil system?

I'll try to post about the Dyneema material later so as not to overload this discussion with too much info.

-Michael

[darkwing 4]

Physicist and rigger here...

Their assertion seems reasonable to me, but could only be determined by actual measurements on the real thing--there are too many important variables that you'd have to estimate. The force on the terminal knot depends on the friction in the system above the knot. It isn't amenable to analysis except very crudely, not in such a way as to cast any doubt or support for their position. Still, their value seems quite reasonable to me.

When tested to failure, I suspect the place the loop breaks is at the business end of the loop, with the reserve pin in it.

Regarding the Vigil system. I have never handled one, so I can't say much, other than it is likely they tested it to failure several times. I don't see the strength at the washer end as being a tricky thing to insure. I suspect theirs also breaks at the pin end.

Jeff

-- Jeff
My Skydiving History

[RiggerLee 61]

All the loops I've seen break under sudden load such as impact snaped at the first turn as it passed through the washer. So is it doing it's job of protecting the knot or is it creating an even weaker stress point at the first sharp bend? I'm not sure it really maters the point is it does it's job, does not seem to be a problem, and would you really want it to be that much stronger any way? At some point you're going to just start to bend the pin and you would probably be better off with the loop breaking at that point any way before some thing else failed.

Lee

Lee
[email protected]
www.velocitysportswear.com

[hackish 8]

I have yet to break one so I really didn't know where they would break. I'm curious to know more about this vigil washer and their closing loop material.

Unfortunately since the vigil allows the cypres material to be used but the cypres does not allow any alternatives, the value of a rigger stocking the vigil kit is somewhat questionable. Unless of course it is much better and there are lots of vigil customers.

-Michael

[riggerrob 561]

Yes, I suspect that a Cypres washer protects the knot by adding friction before the load is felt by the knot.
The key factor is the radius of the holes in the washer.
A small radius acts more like a knife, concentrating tension, while a large radius decreases stress on the closing loop.
Vigil washers have large radii, similar to Cypres washers.

In service, closing loops normally fail at the pin. Much of this failure is attributed to friction and fraying while closing.

[erdnarob 1]

It seems from my point of view that the 2 slots of the Vigil washer play the same role than 2 of the 3 holes of the Cypres washer and this role is to increase friction. I am pretty sure both Vigil and Cypres have made their tests before releasing those designs.

Learn from others mistakes, you will never live long enough to make them all.

[erdnarob 1]

I made an experiment this afternoon: TESTING the breaking strenght of a Cypres single braid.

Here are the testing description and results:

Type of lever: point A (pivot), at 7 cm from A is the point B (where the load is applied or one end of the single braid is) and at 86.5 cm from A is the point C (where the scale is applied)

I used a 50 lbs rated scale equiped with an index staying at the maximun tension applied.

Both ends of the single Cypres braid were secured by a Cypres washer and knots made and placed according the manufacturer info (Cypres manual).

Ratio of the leverage system is 86,5 cm/7 cm = 12.36

The Cypres braid broke when 15 lbs (read from the max index) was applied on the lever at point C

Breaking force of the Cypres cord is therefore: 15 lbs x 12.36 = 185.4 lbs which is different that what was written on the Cypres spool (185 Kp) and in the manual of the Cypres I (180 Kp) I have to admit that I never saw the unit Kp before but somebody in the forum said the meaning was kilogramme pond or Kilogram force. I took it that way first (1 kilogram force = 2.2 lbs) in my precedent posts on the question.

If you double the loop (fingertrap) you get theoretically 185.4 lbs x 2 = 370.8 lbs (on the Cypres II manual I was told that the strenght was indicated at 350 lbs which seems to relate to the already made closing loop. Only Cypres can tell us what they mean by 185 Kp). (single loop or double (fingertraped) loop).

Note 1: There is a confusion on the Cypres spool and in the Cypres I manual as well. it is written 185 Kp and 180 kp while the Cypres loop break at 185.4 lbs in my today's experiment.

Note 2: The break of the single Cypres braid was located exactly flush with the center hole of the Cypres washer placed at the top. The single Cypres braid was more or less vertical while the lever was horizontal at about 90 degrees with resect to the braid.

Due to the lab closing early I will have to return for testing the Vigil Dyneema braid and testing the already made closing loops from the 2 manufacturers

Learn from others mistakes, you will never live long enough to make them all.

[kallend 1,649]

Quote

I made an experiment this afternoon: TESTING the breaking strenght of a Cypres single braid.

Here are the testing description and results:

Type of lever: point A (pivot), at 7 cm from A is the point B (where the load is applied or one end of the single braid is) and at 86.5 cm from A is the point C (where the scale is applied)

I used a 50 lbs rated scale equiped with an index staying at the maximun tension applied.

Both ends of the single Cypres braid were secured by a Cypres washer and knots made and placed according the manufacturer info (Cypres manual).

Ratio of the leverage system is 86,5 cm/7 cm = 12.36

The Cypres braid broke when 15 lbs (read from the max index) was applied on the lever at point C

Breaking force of the Cypres cord is therefore: 15 lbs x 12.36 = 185.4 lbs which is different that what was written on the Cypres spool (185 Kp) and in the manual of the Cypres I (180 Kp) I have to admit that I never saw the unit Kp before but somebody in the forum said the meaning was kilogramme pond or Kilogram force. I took it that way first (1 kilogram force = 2.2 lbs) in my precedent posts on the question.

1.0 kp = 2.205 lbf

...

The only sure way to survive a canopy collision is not to have one.

[kallend 1,649]

If snyone is REALLY interested, we have very sophisticated and accurate mechanical test equipment in our labs. All I need is a few samples and I'll provide all the data you could possibly want.

...

The only sure way to survive a canopy collision is not to have one.

[hackish 8]

erdnarob lives nearby so I can easily get a sample of the material. The scrap metal guy took the fixture I was working on away. That's what I get for leaving it near the door.

After I was finished being pissed off I was considering talking to the local university as they have lots of tensile strength testing machines. If you have open access to a similar machine it may be a viable solution. What sort of attachment would you need to attach the sample to the machine?

-Michael

[JerryBaumchen 1,068]

Hi Aloysius

Send me an email with some contact info for you.

I am wanting to test some low load ( 300 lbs or so ) samples and the test machine I have access to does really work at those low values.

Thanks,

JerryBaumchen