Chockstone Forum

      Sponsored By
      ROCK
   HARDWARE

Chockstone Photography

Australian Landscape Prints

Chockstone Forum - Gear Lust / Lost & Found

Rave About Your Rack Please do not post retail SPAM.

Author

Rope around a tight bend


ajfclark

12-Apr-2010
2:48:15 PM

Reformatted:

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

Am I reading this right that a 10mm rope around a 20mm post = 60% reduction in SWL?

carry

12-Apr-2010
3:00:10 PM

A 20mm diameter post is 2 times your rope diameter.

2d=77% reduction of SWL

carry

12-Apr-2010
3:02:33 PM

I mean SWL is reduced to 77% of what it was originally.


ajfclark

12-Apr-2010
3:07:33 PM

I read that as if the diameter of my rope is 10mm, a post with a radius of 10mm results in 60% strength. That's a 20mm thick post as the diameter is twice the radius. For no strength loss they say an object with a radius 3 times the diameter of the rope, 30mm radius for a 10mm diameter rope. That makes a 60mm diameter post.

carry

12-Apr-2010
3:26:20 PM

I see what you are saying.

I just find it hard to believe there is such a large reduction in strength, but maybe your right.

carry

12-Apr-2010
3:55:36 PM

If I want to rappel with my 9mm rope looped around two opposed caribiners (say a caribiner is 11mm diameter).

22mm (width of two caribiners) / 9mm rope = 2.4d
SWL is about 80% of original strength. I can believe that.

OR

11mm (radius of two caribiners) / 9mm rope = 1.2d
SWL is about 64% of original strength. I think this would shock most people.

And rope isn't making a true round curve over two caribiners side by side either.


IdratherbeclimbingM9

12-Apr-2010
4:10:52 PM

... and 64% of (say) 3000 kg static breaking strain is 1920 kg.
~> Is likely to hold a simulrap with both parties carrying megaloaded wallbags plus some, so no, I am not shocked.
... but what was that anchor creaking sound again?
;-)

Here is a scenario for you carry.
When I have abseiled off a rope doubled over a prussik cord hero looped on a bolt...
The abseil rope is kinked to less than 5mm, ... but what strength was the 5mm prussik cord when kinked under the same load to 2 or 3 mm?
Heh, heh, heh.

Post edit.
Quote from the site you have found.

Use and Care of Ropes
Knots reduce the breaking strength of rope as much as 50%. Energy which is normally spread over the entire length of the rope directs itself to the knot. For this reason, splicing is preferred to knotting.

Sharp bends greatly reduce the strength of a rope. Use padding where possible.

Avoid all abrasive conditions. A rope will be severely damaged if subjected to rough surfaces.

Pulleys shall be free to rotate.

Rope will wear excessively when used with pulleys that are too small. Diameter of pulley groove shall be 1.2 times rope diameter. For nylon and polyester ropes, diameter of pulley shall be more than 8 times rope diameter.

If a rope is used over a round object having radius (known as bending radius r) less than 3.0 times rope diameter (d), reduce safe working load as under.

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

Working Load Limit (WLL) is the maximum allowable capacity in vertical position. Give due consideration to the angle of spread while selecting rope / sling size.

Whenever a load is picked up, stopped, moved or swung, there is an increased force due to dynamic loading. In all such applications, stated working load limit (WLL) by manufacturer should be reduced by 1/3.

The key words in that quote for me, are;
knot = 50% reduction prior the radius thing being applied.
...and
dynamic load = 1/3 further reduction.

~> ... so my initial 1920 kg ends up being 640 kg with those WLL limits applied.

Hmm. A simulrap with my 'scenario' would mean only 320 kg on each strand.
~> Still enough, but now starting to get scarier...

grangrump

13-Apr-2010
1:42:14 PM

I think that the 'radius effect' is the reason that ropes are weakest where knotted.
Still dont think that I'll be doing much kermantle splicing...


IdratherbeclimbingM9

13-Apr-2010
1:53:47 PM

On 13/04/2010 grangrump wrote:
>Still dont think that I'll be doing much kermantle splicing...
>
Most would agree with you. Not because splicing is going (gone?) out of vogue, but because kernmantle ropes (core & sheath), are the newer style;... ie braided (strand), ropes are much easier to splice.

Which reminds me; I wonder if anyone has ever found the manilla sling runners I left on certain climbs on Lion Island many many years ago, as an indication of prior passage for the wonderment of future ascentionists?
Heh, heh heh.

patto

13-Apr-2010
2:05:23 PM

The table figures depend on the type of rope. I would suggest that those figures are incorrect for climbing rope.

I would expect the 60% strength mark to be closer to the r=0.5d level. Otherwise we would be seeing more ropes breaking at bends rather than knots.

One Day Hero

13-Apr-2010
2:06:46 PM

On 12/04/2010 carry wrote:
> I think this would shock most people.
>

Ummm, nothing shocks me...........except girls climbing in shorty shorts which don't reach to the leg loops........and they don't even back up their belay with undies!!!

Man, ropes don't snap at the anchor while people are abseiling!!! Ever!!!

Find one report of it happening and I'll concede that you're not wasting your time with this crap......on the other hand, lots of people die cause they fcuk up their rap setups.....maybe you should find a table for the SWL of a brain engaged in distracting conversation whilst threading a rap!

carry

13-Apr-2010
5:31:52 PM

I found some more info here: http://www.outdoorlink.org/ol/files/trdocs/research-papers/Knot-and-cord-strength.pdf

It's quite interesting reading but this is the significant bit:
" How relevant is tensile strength to loop strength? If you tie it in a loop is it twice the
strength of the rated tensile strength? How much does the carabiner affect the overall
strength? The tensile strength of BW 7mm static cord is 2,600lbf. Loop strength on
carabiner diameter test pins was 4,200lbf or about 80% of double strength. It was
virtually the same percentage in BW 8mm static cord. "

So if looping cord around a carabiner means 80% of original strength, I'm not going to worry about this anymore.

Still not sure about smaller diameters though...

davepalethorpe

13-Apr-2010
10:28:58 PM

Surely the reduced rope strength when it goes around a tight bend cannot be a major problem or otherwise we would have extra wide biners on quickdraws? Pretty sure the diameter of all my quickdraws are equal to or less than the diameter of my ropes.

One Day Hero

14-Apr-2010
2:41:30 PM

.........and the pointless geekery continues. How about looking at it this way? There are lots of engineers working for rope manufacturers who are much smarter, better educated, and more familiar with how ropes behave than you are. These engineers are not freaking out and running round in little circles trying to find out the max sustainable force on an abseil rope through a mallion..........because the answer is that its strong enough for anything you will do to it!

That's it! No number (to 3 decimal places) in Kn, no graph of standard deviations from the mean, no pretty 3D modelling of how cutting proceeds.......because you don't need any of that, it's just "strong enough"! No one cuts their rope over a mallion while abseiling. If they did, the serious people (engineers at rope companies) would be all over it like stink on shit and would come up with serious engineery solutions and recomendations, but they don't so they won't, so think of something else to write about, geek!

carry

14-Apr-2010
4:51:02 PM

Stay constructive and remember you need to take responsibility for yourself and not hope someone else will.

There are 35 messages in this topic.