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Rope around a tight bend

carry

7-Apr-2010
4:03:07 PM

My question is directed to people who use a double rope to abseil and then pull their rope through. Typically you clip the rope mid point to your anchor via two opposed locking caribiners. Seems reasonable.

But what about using a pair of maillons instead? Or what about using a single 6mm maillon (should hold 1750kg)?

Any sharp bend (or knot) reduces the breaking strength of your rope. I want a clearer idea of by how much. I've been searching the internet for days without being able to turn up any real data. Maybe i'm missing something, but this seems odd to me.

Has anyone done stress testing of various diameter ropes looped around various diameter pins?


Phil Box

7-Apr-2010
4:12:01 PM

That would be an excellent series of tests. I'd like to see the tests go further though to see at what stage a rope will cut. In other words start the series at 10mm radius and go down to 1mm radius by 1mm increments and then step down in .2mm increments. This should give us some hard data on how different radii affect the strength of a rope.

Of course this will not give us any data on what would happen when a rope is subject to a sawing action on various frictional edges which is another series of tests I would dearly love to see.

Sooo many things can affect the strength of a rope it is almost bewildering as to where to start which is possibly why these sort of tests have never been done. Do the tests and then throw in a variable, repeat ad nauseum, ad infinitum. Where do you stop. We know where to start but the question remains where one goes with this.


IdratherbeclimbingM9

7-Apr-2010
4:13:41 PM

Selected quote from here.

"There are a lot of publications available that contain information about knots and knot strength. For that reason we will not include much information here, except to say that knots reduce the overall strength of a rope. Therefore, whenever possible use a tensionless hitch around a cylindrical object with at least a 4" diameter. Four inches is the magic number for maintaining full strength in a rope up to 5/8". Any bend tighter than 4" reduces the strength of rope because it stresses the rope in flexure, the fibers weaker direction. Here is a list of some common knots and the translating remaining rope strength:

NO Knot 100%
Double Fisherman's 65-70%
Bowline 70-75%
Water knot 60-70%
Figure 8 on a Bight 75-80%
Clove hitch 60-75%
Overhand 60-65%

This is based on a well tied knot, a knot that has been dressed. If the knot is poorly tied and is not dressed, the strength of the rope will greatly decrease from the numbers above.

Regarding knot strength. Here is another link if you want to go down that track...

Is your question practicality based or research based?

I have abseiled on a rope doubled over a bend of 5mm (single maillons, sometimes other assorted crappy leave-behind accessory krabs rated to 650 kg, and sometimes thin prussik cord hero looped to a bolt), semi-often(!), with no 'apparent' ill effect, ... yet... heh, heh, heh.

Post edit:
Some interesting fatigue testing of 'ropes' carried out here.


nmonteith

7-Apr-2010
4:18:44 PM

On 7/04/2010 carry wrote:
>My question is directed to people who use a double rope to abseil and then
>pull their rope through. Typically you clip the rope mid point to your
>anchor via two opposed locking caribiners. Seems reasonable.
>
>But what about using a pair of maillons instead? Or what about using a
>single 6mm maillon (should hold 1750kg)?

I'm confused. I tie my two ropes together using a fishermans knot at the mid-point rather than clipping them together. Isn't that the normal practice when rapping with double ropes?

citationx

7-Apr-2010
4:29:08 PM

On 7/04/2010 nmonteith wrote:
>On 7/04/2010 carry wrote:

>I'm confused. I tie my two ropes together using a fishermans knot at the
>mid-point rather than clipping them together. Isn't that the normal practice
>when rapping with double ropes?

I assume when they refer to "clipping the rope to midpoint" they're alluding to a situation in a top roping scenario, where (for quick and ease of setup) you'd clip some biners to the anchor and then clip the rope through the biners (as opposed to what we'd normally do when rapping off a double bolt - that is feeding one entire rope through until it hits the knot between the two ropes), and then referring to rapping on an anchor that is solely a maillon, and the diameter of the maillon.

If that's the case, i'd be happy rapping off a piece of metal the diameter of a maillon. You can argue that rope sharp tests are done on 0.8mm edges and they take have to withstand a factor 1.6 fall on that (or some scenario similar), so i reckon that a 6mm maillon is plenty thick to not cut through your rope...
There are plenty of examples of anchors in the blue mountains where one is a twisted shackle or a ring and the other point is a maillon. Sure, in normal blueys climbing i'd definitely lower off two anchor points, i guess I would still be happy to rap off the one point in "real mountains" however i'd just abseil more slowly/smoothly/carefully...


nmonteith

7-Apr-2010
4:38:18 PM

I've rapped directly off fixed hangers in the past. Doesn't seem to do any obvious damage to the rope. Providing you keep the force static (ie don't lead fall on it!) I think small diameters are fine.

Wollemi

7-Apr-2010
4:48:14 PM

I was going to mention that a single non-tubular sling in Blue Mts canyons or otherwise would create a tighter kink in the rope than a 6mm maillon.
But Neil's response betters mine.


IdratherbeclimbingM9

7-Apr-2010
5:44:11 PM

On 7/04/2010 carry wrote:
>I've been searching the internet for days without being able to turn up any real data. Maybe i'm missing something, but this seems odd to me.
>
>Has anyone done stress testing of various diameter ropes looped around various diameter pins?

Lots of links
... I stopped at page 4 without looking at any of the wire rope links, and found some of them contained quite a lot of facts and figures for diameter/fatigue tests, safety-manual/handbooks, etc.

Some relevant quotes from some of the sources...
http://www.ropeinc.com/rope-inspection-retirement.html
BENDING: Any sharp bend in a rope under load decreases its strength substantially and may cause premature damage and failure. Sheave diameters on rotating sheave blocks should be 10 times the rope diameter for twisted ropes and 8 times the rope diameter for braided ropes. The diameter on fixed pin terminations should be at least 3 times the rope diameter (i.e. the bending radius for 1/2" ropes should be 1-1/2").

http://www.vectranfiber.com/bend_tolerance.asp

http://www.vectranfiber.com/pdf/19Pages%20from%20Vectran_broc_final61206-11.pdf

Bend Tolerance
Tolerance to bending around small radii is important in ropes and cables, as it allows the use of smaller running gears or termination hardware. Aerospace and rope manufacturers conducted pin diameter tests on Vectran™ braid and wire rope, respectively. The test configurations are shown in Figure 17.
For the braid tests, each sample was 30 inches long and eye spliced on both ends with a long taper to minimize stress concentration where the splice begins. Each sample was tensioned three times to half its breaking strength to remove construction slippage before being tensioned to break. Pin diameters ranged from 0.110 inches to 0.31 inches. D/d (pin diameter/rope diameter) ranged from 1.5 down to 0.7.

Handbook of Rigging: Lifting, Hoisting, and Scaffolding for Construction and ...
... lots of facts and figures.

~> and if you are not totally bored yet, here is some more light reading!

http://allaboutknots.blogspot.com/2006/01/selecting-strong-hitch-for.html

How to Increase the Strength of a Knot

The factors that affect knot security and stability are fairly straightforward, but those that affect knot strength are more complex. To begin with, you need to recognize that knots usually break because an excessive load falls on the first curve; the severity of other curves in a knot do not affect its strength. You can increase the strength of a knot in two ways: 1) reduce the load on the first curve; 2) reduce the severity of the first curve. Either of these ways reduces the stress on the first curve. Increasing the number of round turns made through the ring or shackle does not increase the knot’s strength unless it also makes the first curve more gentle.

How to Reduce the Load on the First Curve

It is possible to reduce the load on the first curve only if the stem curves for the first time inside the nub of the knot, such as in a Double Fisherman's Knot.

How to Reduce the Severity of the First Curve

The second way to reduce the stress on the first curve is to make the first curve gentle. A knot that creates only a gentle curve at the point the stem first begins to curve will be strong. It is particularly important that the hitch does not make a severe curve as it enters the nub.

The obvious way to reduce the severity of the first curve is to use a knot in which the first curve occurs as the rope passes through its mooring, then to increase the diameter of the ring or shackle. This would reduce stress on the rope and thus increase its strength. You could do this by inserting a thimble in the ring or by increasing the diameter of the sheave in a block. Neither of these solutions may not be practical in the application specified here. In these tight situations, probably the best thing to do is to choose a knot in which the first curve is located at some point other than the place the rope passes around the ring.

carry

7-Apr-2010
7:29:45 PM

Thankyou for all the links. I have been through most of these already. There is a lot of info around about how rope strength is compromised from different types of knots, but my question is not about that.

It's more like this example:
If I have a 9mm static rope (breaking strength 23kn) and I loop the midpoint around a 7mm maillon anchor. What weight, hanging off both rope ends will cause the rope to break? I'm assuming the rope will break where it it bearing on the maillon. And does the diameter of the maillon make a big difference? I'm not sure, but I think it would.

One of your links says: "The diameter on fixed pin terminations should be at least 3 times the rope diameter". This sounds like good advice but I want more specific data.

You know something like:
X diameter static rope looped once around Y diameter pin = Z% of normal rope breaking strength

Duncan

7-Apr-2010
7:41:46 PM

I very highly doubt that there's any chance of your rope breaking. Think about it. Even if looping your 23 kN rated rope over a 7 mm maillon reduces the breaking strength of your rope by 99 %, it's still good for 0.23 kN. Or, as I understand it, about 230 kg of static force. Do you weigh more than 200 kg?

egosan

7-Apr-2010
8:52:10 PM

Aye, quit fussing. Go climb.
Your question is legitimate.
However soon as you start converting KN to Kg you will see you don't have much to worry about.
However don't let this mislead you about redundency in your anchors. The failure modes of anchors very different to the failure modes of ropes.

carry

7-Apr-2010
9:29:03 PM

Yes, I should be more concerned about the strength of my anchors rather than my rope snapping in two.

Still I can't understand why I can't find tables on such a basic breaking strength test. Will let you know if I find out more about this.

dave

7-Apr-2010
10:24:13 PM

On 7/04/2010 Duncan wrote:
>I very highly doubt that there's any chance of your rope breaking. Think
>about it. Even if looping your 23 kN rated rope over a 7 mm maillon reduces
>the breaking strength of your rope by 99 %, it's still good for 0.23 kN.
> Or, as I understand it, about 230 kg of static force. Do you weigh more
>than 200 kg?

That doesn't sound right.
0.23kN is around 23kg of force not 230...So actually you'd be dead!

egosan

8-Apr-2010
6:45:27 AM

Dave is definately better at maths than Duncan. Still I believe to get the rope down to 1% strength you would have to rap off my back up belay device.

Wendy

8-Apr-2010
8:33:16 AM

On 8/04/2010 egosan wrote:
>Dave is definately better at maths than Duncan. Still I believe to get
>the rope down to 1% strength you would have to rap off my back up belay
>device.
>
>

>/img>

After cutting old ropes to replace anchors in Thailand, I'm almost convinced you could rap off that. It's bloody hard work cutting rope with a pocket knife.

PDRM

8-Apr-2010
9:59:27 AM

>After cutting old ropes to replace anchors in Thailand, I'm almost convinced
>you could rap off that. It's bloody hard work cutting rope with a pocket
>knife.

Not once you put some weight on them...

Paul

Duncan

8-Apr-2010
10:30:17 AM

Oops, fair enough. You're right, it didn't sound quite right when I wrote it. Make it a 90 % reduction in strength, the idea still holds - you're not going to die rapping off a maillon.


nmonteith

8-Apr-2010
10:54:39 AM

Every sport climber has probably bailed off a single 6mm mallion at some point in their climbing life. If you lower-off its putting twice the weight on the mallion that if you rapped off. Its bomber.

grangrump

8-Apr-2010
11:27:28 AM

On 8/04/2010 nmonteith wrote:
>Every sport climber has probably bailed off a single 6mm mallion at some
>point in their climbing life. If you lower-off its putting twice the weight
>on the mallion that if you rapped off. Its bomber.

And any sport climber has lead fallen on a rope going through a biner = small radius,
i.e. putting way more force on the rope than any rap
(and yes rope slippage during fall may spread point loading somewhat, but not to the point that I'd worry about the rap)

carry

12-Apr-2010
2:17:42 PM

I found what I was looking for on: http://practicalmaintenance.net/?p=310

Bending radius “r” d 1.5d 2d 2.5d
Reduction of safe working loads to 60% 70% 77% 82%

d is the rope diameter.

Interesting.

There are 35 messages in this topic.