Basic Rope Technique

[JMJones0424]

Thanks for persistently pursuing my ambiguity.

 

Especially for light loads in practice, as long as you are protecting the prusik loop from the friction of pulling the line through, I think anything would be ok.

 

I have broken 11mm static kernmantle rappel line on two occasions, both using a z rig, and both while using the line in situations that I really should have known better. Also, on both occasions, the line snapped while tightening and in the portion of the line under the most tension (as shown in post 6). Tensile strength for static line varies, we carried 7500 pound minimum in the Marines, and I think the line I have now is rated at 7200 pounds. The "safe" load is 1/5 the rating of the line, but this rule of thumb is to protect against additional load from a fall and other factors that affect the line (like using a line while it is wet).

 

First occasion was while trying to cross a narrow but deep stream. We had gear in our packs protected from light water exposure (like rain), but our packs were not water proof and we would have ruined our gear if we had swam across, not to mention the packs would have likely sunk. We sent two swimmers with one end of the line and prusik loops and carabiners across and fixed the other end of the line up high on a tree on our end of the bank. The plan was to use a z rig to tighten the line enough that even sagging under the load of packs, the packs would remain above water. I don't remember how many "floating" pulleys we used, but I distinctly remember the disappointment when the line snapped and we realized we had to find a different way to get across.

 

On the second occasion, a friend (no really, it wasn't me I swear :)) drove his truck into a ravine, front first, with the rear wheels spinning freely mid-air. The plan was to pull the truck out far enough to get the drive wheels back on the ground and back out. The nearest tree was further than the length of chain I had, and I had no come along, so I tried a z rig and snapped the line there too.

 

Moral of the story is: If your load requires you to use more than three or so "floating" pulleys, then you need to double the line.

 

it seems to me that besides the added friction of carabiners vs. pulleys, the sharp bends of the rope in carabiners is bound to reduce the strength of the line more than the line bends around a pulley. while i can find plenty on the amounts that different knots & splices reduce a line's strength, i can't find anything on the diameter of a bend in a rope affecting tensile strength. is it possible that all the knots & sharp bends in your breaking rig contributed to the failure?

 

I do not know. Splicing a line is not something I have ever done, but the weakness seems pretty self-evident there. (Caveat: All the following comes from what I have been taught. I have never performed failure tests on rappel line and have not seen anything that would either confirm or disprove what I was taught.) When tying a knot in rappel line, the reduction in tensile strength is due to the way the knot compresses down on the line, preventing stretching. When tying complex knots, it is extremely important to make sure that the knot "lays" correctly and that there are no internal twists that prevent the knot from working properly. The figure eight loop is an excellent example of a knot that can be greatly weakened, even when tied "correctly", if there are improper twists in the knot. Examples abound on the internet, here's a good one (ironicly, from an article entitled "Proper Rope Tying for Rock Climbing"):

 

Source

 

The red arrows point to the twist, the knot will fail where the arrow is pointing to the left, because the line is twisted and will place too much stress where the arrow points down.

 

Do sharp bends around a carabiner produce more strain on a rappell line than the same line going around a pulley? I'm not sure, but I don't think the difference would be significant. Both of the times a line has failed on me, it has been along a straight run of the line, not around a carabiner. This doesn't mean that the damage that led to failure didn't occur as the line went through a carabiner though.

[Tormod]

Fantastic topic. Thanks!

[Turtle]

Fantastic topic. Thanks!

 

:thumbs_up

 

still waiting for the weather to make like a rope & break so i can try some rigs out. :weather_rain:

 

in the mean time i am still looking for info on how the "tightness" of a bend in a rope around an object affects the strength. nothing on fabric ropes yet, but i found something on wire cables that seems to hint at what i am getting at.

 

Care & Use of Wire Rope Slings

 

...To illustrate, each sling leg in a vertical basket hitch absorbs 500 lbs. of stress from a 1,000 lb. load. The same load, when lifted in a 60 degree basket hitch, exerts 577 lbs. of tension on each leg.

It is critical therefore, that rated capacities be reduced to account for sling angles. Angles less than 45 degrees are not recommended and those below 30 degrees should be avoided whenever possible. Use the formula and chart shown below to calculate the reduction in rated capacities caused by various sling angles.

 

 

anyway, gotta cut away for a bight of breakfast. :doh:

[Turtle]

santa had to back-order a carabiner for me according to elves, but i have their assurances. :xmas_sheep: in the mean time i made up another loop & worked up a couple quick-release-square-knot-overhand-backed-prusiks. i made the second loop -lower knot in pic- 4 1/2 feet long rather than 4 feet to take up some of the overhand shortening.

[Turtle]

...As far as mechanical advantage goes, I think I have been incorrect. According to the wikipedia entry on the z drag that I posted, it has a mechanical advantage of 3:1 (assuming zero friction). The second z drag that I posted (and assumed was 3:1) would actually be 6:1. From the same site as the prusik minding pulley image:

 

I have no way of knowing if those figures are accurate, except that I think it is more correct to say the second rig I posted is roughly twice as efficient as the first rather than 50% more efficient. This means the rig I assumed was 5:1 might actually be 12:1. I honestly don't know, I'll have to let you do the math if you wish, or I can break out my ropes on my next day off and do some measuring. I hope these diagrams are a little easier to understand than mine. The second rig I posted originally on 7 Dec that I said I would use to pull up a load by myself corresponds to rig "E" in the image above.

 

some more questions & no answers. :hihi: so i don't know how to calculate the mechanical advantages in those last 4 diagrams, as they do not match up with what i have read on pulleys. (or at least i haven't been able to make the matchup. ) a case in point, and what got me thinking, is that i read that in an ideal system the mechanical advantage & velocity ratio are equal. while i did say i read that the velocity ratio is the number of ropes supporting the load, what i read & didn't write is that the velocity ratio gives how many feet of rope you must pull to move the load 1 foot. for example with a velocity ratio of 2:1 you have to pull 2 feet of rope to advance the load 1 foot. so my question is does this hold for the rigs you have used. for example in the E rig, do you have to pull 6 feet of rope to move the load a foot? moreover, do you think that just by measuring the velocity ratio we could say what the mechanical advantage is? that is to say we measure how many feet of rope we pull and how many feet the load moves.

 

that's all i got. . . .

[JMJones0424]

I am assuming that if we ignore friction, velocity ratio = mechanical advantage. When I tested the rigs in that diagram, I tested velocity ratio, and found the reported values to be correct. Again, the difference between the z drag and a standard block and tackle is the existence of the "floating" pulleys (not just the moving pulleys attached to the load). These are the pulleys that are attached to moving parts of the line instead of the anchor point or the load. I know of no industrial application for this concept, as it has its limitations. Using standard block and tackle, one can lift the load all the way to the top without resetting the rig, this is impossible with a z drag. Also, with standard block and tackle, the weight of the load is evenly divided between all the lengths of line between the moving pulleys and the fixed pulleys, where a z drag (depending on the exact setup) has portions of the line that bear more weight than others. The main advantage of the z drag is the ability to gain more mechanical advantage with less line and pulleys.

 

Note that swapping out the "anchor" position with the "load" position DOES affect the velocity ratio. Some of my first posts in this thread were incorrect on that account as well.

 

Referring back to the same diagram, figures A-C have no "floating" pulleys. (The pulley in C is attached to the line coming off the load, so it is equivalent to having the pulley on the load.) Starting with figure D, a "floating" pulley is introduced. What would have been a 3:1 standard block and tackle is pulled by a 2:1 standard block and tackle, giving a final velocity ratio of 6. Figure E is a 2:1 pulled by a 3:1. Figure F is a 3:1 pulled by a 3:1. This is what leads me to believe the use of "floating" pulleys leads to multiplicative rather than additive relationships. If I need to do some photoshopping to illustrate my point better, let me know and I will.

 

BTW, the quick-release prusiks look to be spot on. :)

[Turtle]

...

Referring back to the same diagram, figures A-C have no "floating" pulleys. (The pulley in C is attached to the line coming off the load, so it is equivalent to having the pulley on the load.) Starting with figure D, a "floating" pulley is introduced. What would have been a 3:1 standard block and tackle is pulled by a 2:1 standard block and tackle, giving a final velocity ratio of 6. Figure E is a 2:1 pulled by a 3:1. Figure F is a 3:1 pulled by a 3:1. This is what leads me to believe the use of "floating" pulleys leads to multiplicative rather than additive relationships. If I need to do some photoshopping to illustrate my point better, let me know and I will.

 

BTW, the quick-release prusiks look to be spot on. :)

 

bingo!! that phraseology makes pefect sense to me now. thanks. :) thanks too for checking my knots as i may well be using them & it's knot the kind of thing i want to get wrong in the field.