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Distribution of Force
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stoneguy


Jan 27, 2013, 5:09 AM
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Distribution of Force
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In recent discussions with a rigging manufacturer, I have some lack of understanding.
A 1/8" stainless wire(cable) is rated at 1760 lbs.When doubled thru a nut it theoretically becomes 1760 x 2 at 85-90% of it's strength or 2992 lbs. The crimp/swage should also be 85% of the load.

When climbing (or falling) on a rope, there would be a 5Kn force(as an example) on one side and an equal an opposing 5Kn force on the other, thus creating double the force on the anchor/nut.(or 10Kn)

If this theory is correct , then why does it not apply to the rating of the nuts we use.?

And, yes, I am assuming I have missed something, as the manufacturers of our gear should be correct.


JimTitt


Jan 27, 2013, 5:45 AM
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Re: [stoneguy] Distribution of Force [In reply to]
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YouŽre definately missing something! IŽm not sure what it is though since the theory you explained is fundamentally correct.
The strength rating on nuts isnŽt always related to the wire as they are tested as a single component and the nut may collapse before the wire breaks especially in the smaller sizes.


wivanoff


Jan 27, 2013, 6:18 AM
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stoneguy wrote:
When climbing (or falling) on a rope, there would be a 5Kn force(as an example) on one side and an equal an opposing 5Kn force on the other, thus creating double the force on the anchor/nut.(or 10Kn)

I thought it was NOT double because of "pulley effect" and friction over the carabiner? http://www.southeastclimbing.com/faq/faq_pulley.htm

For nuts, maybe the difference for swaged cables on nuts is because manufacturer's list the lower number to cover themselves? I suspect that in the real world, for any nut with 1/8" cable, the placement is more important than the theoretical breaking strength of the cable.


stoneguy


Jan 27, 2013, 6:24 AM
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A sling would equalize much faster than a hard inflexible 719 strand wire, so it might not distribute weight the same way. My guess is that in the end, the manufacturer simply tests it under load and "calls" it at that.
Some of my concern is the move to non-standard countries who may not test the same. Some of the wire I measured was .105-.110 which is curious. American tolerance would be plus something minus nothing, so .110" indicates maybe 1/8" plus OR MINUS.

If that force somehow got applied to one side of the wire (which I guess is doubtful) then it could be 1200-1500 lbs on this apparent 10Kn nut/wire.

I assume the manufacturers would continue to accurately protect us.


wivanoff


Jan 27, 2013, 6:49 AM
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stoneguy wrote:
Some of the wire I measured was .105-.110 which is curious. American tolerance would be plus something minus nothing, so .110" indicates maybe 1/8" plus OR MINUS.

Or it could be nominal 3mm which is .118"


dan2see


Jan 27, 2013, 8:13 AM
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stoneguy wrote:
In recent discussions with a rigging manufacturer, I have some lack of understanding.
A 1/8" stainless wire(cable) is rated at 1760 lbs.When doubled thru a nut it theoretically becomes 1760 x 2 at 85-90% of it's strength or 2992 lbs. The crimp/swage should also be 85% of the load.

When climbing (or falling) on a rope, there would be a 5Kn force(as an example) on one side and an equal an opposing 5Kn force on the other, thus creating double the force on the anchor/nut.(or 10Kn)

If this theory is correct , then why does it not apply to the rating of the nuts we use.?

And, yes, I am assuming I have missed something, as the manufacturers of our gear should be correct.

When you fall on a top-rope anchor or a pro, the force of your stop is applied to the strand you're tied to. The pulley at the anchor requires a balance, and that comes from your belayer's strand. That's how "rope", "pulley", and "twice the force" show up in the same sentence, and your 5 kn becomes 10 kn.

Well, if that 5+5=10 kn is applied to a single nut, the nut gets 10 kn, the rock gets 10 kn, and the wire's carabiner gets 10 kn. It doesn't matter what material the nut is made with. Anyway your nut has 2 wire strands each wire strand gets 5 kn. I don't think it's possible that the two wire strands can equalize dynamically.


JimTitt


Jan 27, 2013, 9:29 AM
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stoneguy wrote:
A sling would equalize much faster than a hard inflexible 719 strand wire, so it might not distribute weight the same way. My guess is that in the end, the manufacturer simply tests it under load and "calls" it at that.
Some of my concern is the move to non-standard countries who may not test the same. Some of the wire I measured was .105-.110 which is curious. American tolerance would be plus something minus nothing, so .110" indicates maybe 1/8" plus OR MINUS.

If that force somehow got applied to one side of the wire (which I guess is doubtful) then it could be 1200-1500 lbs on this apparent 10Kn nut/wire.

I assume the manufacturers would continue to accurately protect us.

YouŽd be suprised how badly a sling equalises the load and how well a 7/19 thin wire does, though exactly which 7/19 wire you choose might make a difference (the numbers denote classes of wire and not the exact number of wires which will vary depending on what the cable is designed for).
There is an allowed tolerance for wire measurement which is -0% + 5 to 8% depending on the wire diameter.
There is a "soft" metric conversion system for imperial wire so the diameters may not be anything you recognise.

Yes, the gear manufacturers generally know what they are doing. Wire failure of new gear is unheard of apart from faulty terminations.


stoneguy


Jan 27, 2013, 11:35 AM
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Even the .118 should be that or higher.


stoneguy


Jan 27, 2013, 11:40 AM
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Yes, it's a bit of a conundrum.


patto


Jan 27, 2013, 6:59 PM
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Why assume that a nut breaks at the swage?

Countersunk DMM nuts are rated to 12kN.


JimTitt


Jan 27, 2013, 11:06 PM
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wivanoff wrote:
stoneguy wrote:
Some of the wire I measured was .105-.110 which is curious. American tolerance would be plus something minus nothing, so .110" indicates maybe 1/8" plus OR MINUS.

Or it could be nominal 3mm which is .118"

Or 3/32" at the top of the tolerance (0.106) or 7/64" (0.109) or some innacuracies in the measurement!


stoneguy


Jan 28, 2013, 4:14 AM
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I'm actually not asuming very much. Mostly, I just don't understand the math application.
It is my limited understanding that the wires tend to break not at the swage, but about 1/3 up from the bottom on the opposing side. Theoretically I would think the swage to be the weaker, or according to the rigging manufacturer 85-90% capacity of the wire.
I have seen so "little" information, that it would be hard to make any judgements.


JimTitt


Jan 28, 2013, 5:24 AM
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 "out of the 21 wires tested, 12 broke at the top bend and six at the bottom bend" -Dan Commander testing old wired nuts at DMM

There is also "Wire cable failures in climbing anchor chocks." Advances in Fracture and Damage Mechanics VI, 348-349, 165-168. Vogwell, J., & Minguez, J. M. (2007) which covers the matter in laborious detail.


viciado


Jan 28, 2013, 5:42 AM
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Since the wires tested broke at the bend(s), a reasonable hypothesis seems to be that the bend compromises the strength more than the swage. This also appears (to me anyway) to be consistent with what we know about cord and webbing strength in relation to bends, knots and running over small radius edges.


bearbreeder


Jan 28, 2013, 12:15 PM
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its all theoretical till it actually happens to you Wink





viciado


Jan 28, 2013, 1:06 PM
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Better damaged than failed!

You gonna get somebody to pull test that to failure now?


bearbreeder


Jan 28, 2013, 1:19 PM
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im thinking of taking more whippers on it ... backed up by a million pieces of course Wink


Partner cracklover


Jan 28, 2013, 3:01 PM
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stoneguy wrote:
In recent discussions with a rigging manufacturer, I have some lack of understanding.
A 1/8" stainless wire(cable) is rated at 1760 lbs.When doubled thru a nut it theoretically becomes 1760 x 2 at 85-90% of it's strength or 2992 lbs. The crimp/swage should also be 85% of the load.

When climbing (or falling) on a rope, there would be a 5Kn force(as an example) on one side and an equal an opposing 5Kn force on the other, thus creating double the force on the anchor/nut.(or 10Kn)

If this theory is correct , then why does it not apply to the rating of the nuts we use.?

And, yes, I am assuming I have missed something, as the manufacturers of our gear should be correct.

What are you asking? I don't see a question.

GO


patto


Jan 28, 2013, 9:08 PM
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bearbreeder wrote:
its all theoretical till it actually happens to you Wink


[image]http://www.mountainproject.com/images/79/83/107977983_medium_ffb020.jpg[/image]

That is exactly where you would expect to see extreme fibres break due to a fall when the tension isn't perfectly aligned with the strands exiting the nut.


curt


Jan 28, 2013, 9:43 PM
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cracklover wrote:
stoneguy wrote:
In recent discussions with a rigging manufacturer, I have some lack of understanding.
A 1/8" stainless wire(cable) is rated at 1760 lbs.When doubled thru a nut it theoretically becomes 1760 x 2 at 85-90% of it's strength or 2992 lbs. The crimp/swage should also be 85% of the load.

When climbing (or falling) on a rope, there would be a 5Kn force(as an example) on one side and an equal an opposing 5Kn force on the other, thus creating double the force on the anchor/nut.(or 10Kn)

If this theory is correct , then why does it not apply to the rating of the nuts we use.?

And, yes, I am assuming I have missed something, as the manufacturers of our gear should be correct.

What are you asking? I don't see a question.

Or a problem.

Curt


bearbreeder


Jan 28, 2013, 10:05 PM
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patto wrote:

That is exactly where you would expect to see extreme fibres break due to a fall when the tension isn't perfectly aligned with the strands exiting the nut.


its an offset ... the break happens at the kinks that you get from cleaning

ive fallen from a DMM peanut in the exact same placement many times without much issue ...

the green (its been fallen on so much its barely green) and gray peanuts have taken 10-20 falls each ... and the large brass offset in this pic has only taken 1 or 2 ...

yet you can see the difference Wink




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