Weaker Tensile Rope for Safer Rigging

[TheTreeSpyder]

Should you use a 'bull line' or 'old climbing line' (or other 1/2") for standard rigging (200-400#?)? A while back Mike popped forward a statemeant about using an even lighter line for rigging smaller stuff etc.

According to the numbers run through the Sherrill Rigging Software ; a point comes back up, only stronger than usual to me.

There are more forces in rigging than most have ever imagined. These immense, immense forces must go someplace. The line is comprised of 2 properties mainly beyond just it's amount/length. The line has tensile and elasticity to deal with the immense, immense loads of rigging. If the immense, immense forces cannot take advantage of the flexability,they will raise the line tension; which as if the power was electricity, lights with the increased power everything it touches in the line.

This includes the Porty and Pulley positions, the Pulley/Redirect of Porty Brake Force taking up to a double dose. These variances can make a differance in tons of force on the Pulley position from same load setup!! The only differance being rope type and length employed. The given elasticity of the line, directly handles the task. So a more elastic line will do it better, the dampner in the line handles the force. But, the amoutn of elasticity the line gives of it's properties must be gotten from the line by 2 ways: the lengt of the line/assembled elasticity of a particiular line to a higher value. And, by matching the load to closer to the line's strength, whereby the amoutn of elasticity is accessed by the % of tensile strenght of the line used! Whereby, of the same material; weaker and longer both give more dampening of the immense, immense forces!!

Or sumetin like that!
:alien: :alien:

 

[Stumper]

Uh huh. Did I hear someone advocate taking smaller pieces as a standard practice?

 

[TheTreeSpyder]

The Setup scenario that the software was in when i started the tests turned out a lil obtuse. 5' from CG to pulley, 22' of line as i look at the settings now, but the pattern of the lesson, though perhaps dramatized by the settings some, i beleive remains the same.

If the line flexability doesn't take the immense, immense forces, then through line tension that is transfered to rigging points touched by the line power/tension; double loading at pulley, where perhaps a man is, or near. Through these examinations, taking a bigger load than mentioned on the smaller line can be safer than 100# on the strongest, less dynamic, shorter amount of elasticity exposed in length of the linking unit of line. Like a fanbelt link to motor would take buffer more shocks, siezing of lawnmower blade in high grass to motor (softer, slip link), and a direct drive of all metal could transfer more directly the shocks/power of blades and motor on either side of it's linking. The line properties and amount of properties exposed (through lenght and % of tensile loaded) is this link choice.

This means there is mucho differance between tethering a saw to your hip with a 3/4"StableBraid vs. a 200# tensile nyon line; by virtues of elasticity and % of tensile loaded; assuming matching laods and leash lengths, saw dropped from just as high etc.

 

[Stumper]

All this points to the dangers of overloading rigging with large loads along with the real possibility of overloading the tree/spar/limb we are working off of when using "super ropes". Different set ups and scenarios require different ropes. Most of us are choosing low stretch rope materials (polyester with a few super braids thrown in). The advantage (and it can be important) of low stretch is that loads don''t sag right on down to a roof or other target. The disadvantage is greater shockloading throughout the system. For many years I rigged out pines on 3 strand semi-hard lay nylon (7/16 Mil spec rappelling rope) That rope was long wearing, sap resistant, ran well on 1/2 inch pulleys and was/is somewhat dynamic-it doesn't stretch much with loads up to 200 lbs but after that it stretches more and more. It worked quite well for most removals-but when bigger blocks were being cut near roof level I had to switch ropes-otherwise it might have allowed contact due to stretch. Having some give in the rope was helpful at other times-it allowed a neophyte groundman to lock off loads instead of running them without nearly as much shock to the tree.

 

[TheTreeSpyder]

i call it Mountain/Rescue one up on us again with their more dynamic lines, and choices to task.

The cost of more raw power for clearance, can be more expensive as anything else, ony here it comes at higher forces, unless, we do the work ourselves and pretighten the lighter /more elastic lines i think. Sweating in once again; for pretightening is just doing that part of the work of matching the line tension to the load force(?); yourself, earlier, as a by product, reducing the shock loading; thereby componding the positive affects of pre-tighhtening by reducing the amount of force to match the load force; and doing part of that reduced work yourself, rather than Nature's unrolling events. Then, the bends on load, make tension an operative thing that is useful, to not worry about the higher loading, or as a companion to reducing the loading , then using whatever is left of loading to use to target; while the pretensioning amount also empowers the torque bend more immediately and leveraged, by the higher tension on bent line at first movement(?); in a tandem operation of reducing the force, yet using what you do have in the bend to target i think.

Or something like that.
:alien:

 

[glens]

Ken, the numbers sure look scary.  What bit of information are we missing?  Certainly there will be more shock load arresting a free-falling object the more time it's had to accelerate.  What sort of initial free-fall factor is being used in the calculations?  All this stuff would make more sense if for some reason you were working alone up on the tree, but it seems good practice to put the long end of the rope in someone's hands and that they'd take up appropriate slack.

Aren't there really too many variables to use the software for anything but to find possible worst-case (as when both ends of the line are firmly affixed) values?

Glen

 

[TheTreeSpyder]

Hey Tire Bait!

Just as you/we both say elasticity in the line can help on vehichle pull, when others would use chains, here too the elasticity factor can be a working element.

If trying to limit/take larger and still limit force on supports, and switch to fat line may in some instances be a bad decision, as less of the tensile strength will be tested of the line, so less elastcicty 'squeezed' from it to dampen the forces!!

Mike said a while back about using a light, easier to carry line for small loads; here it is less fatiguing to carry, and loads stuff less! All just pieces to the puzzle.

i think the main lesson, is that the peak forces are present, and must be dealt with by the system. If the lowering is controlled, and the elastcicty of the line isn't used to dampen the force, the higher line tension passes the force to the supports instead, for it still must be dealt with.

Sometimes it will matter, sometiems it won't; when it does, i think the understanding and imagery lends light.

The software isn't perfect, overhead rigging, pretensioned lines etc. aren't covered. But the pattern, of the outside peak forces, sets the constraints for the rest of the forgiveness in the system to take away from the high values of force presented. So the softwre provides the shell to work from i think; but still patterns to keep in mind etc.

Also, placing a pulley on load for 2/1 w/drop eye pulley or similar all on same support will lower the standing load on the support over a 1/1 pulley redirect off of support, no pulley on load. but in the dynamic scenario, as 1/2 force is placed on each leg of line, less of the tensile strength is challenged and less elastiocity in legs of support in response, for much higher dynamic loads on 2/1 mvoing, even though less standing.

i think the point is it is not about knowing enough to drown the situation in overkill of SWL; but knowing enough confidentally to have a lower SWL (but still safe) to draw out more of the line's elasticity; for minimal loading examples from the same load. That once again it is not about overwhelming force, but finesse; the recomendations might not say 25x1 SWL for a reason?

 

[TheTreeSpyder]

Another lesson in the mess here; and one of the questions i always wnated to get into with the software; was the question that has gone around here a few times about force of a dropped saw, tied to your hip. Right off the top; after messing with software, it can be seen that it is best/most comfortable, if we can select a line/product that has a low tensile/ lots of give in the 7# range(or whatever). An 8100# piece of Blue Streak is not a good choice. i would look for 2 things if making own from line only, or shopping (considerations of stitch tearout aside and better, mebbe tuned for load). i would seek material with good flexability, and something that could hold saw, but not be 8100# strong, the 7# example would be less than .1% of the tensile of the line. There is real force seen here now, beyond what suspected. i would think we would want to use/'abuse' the rope by choosing a much lower tensile (that 7# would be a higher % of, there for 'use' the rope more) and not stiff. i would want whatever force to be as dissipated as possible by using the rope's elasticity. Starting with a choice that is more dynamic is best, then by material /construction; then a strength more matching the work, so the line loads and works, giving the effect of a slight bungee snubber to line force peaks, vs. stiff, jarring cable like , direct shocks of the peak forces.

 

[glens]

I can only imagine catching a dropped saw by a rope attached to my hip.  Assuming enough "rope" is used to allow unobstructed reach/use of the saw in any possible (safe) situation and a saw weighing 8 lb. I'd guess 50 or 60 lb snatch, max, if the fall is completely vertical from the highest point available, but less in practice (I have not read any of the aforementioned threads).  This is a different situation though, than having a co-worker snub a just-cut limb/whatever.  Here, we have a fixed length of rope firmly attached at both ends; all give (if any) will be totally within the rope (or at least between the attachments).  There, the rope may be wrapped at the bottom end as well, but there's someone who presumably will eat/feed out material, if even the most slightly, at the right times to regulate things dynamically.

Glen

 

[NeTree]

So, Kenny...


How long does it take you to do a removal?

 

[a_lopa]

i really think you should be a crane driver kenny.100tonne all terrain

 

[murphy4trees]

I think Spidy has uncoverred another gem here for all of us to benefit from.
I was surprised years ago to find out that fall factor is considerred much more important than distance fallen in rock climbing...

The point is: the force in shock loads can be huge and are best regulated by stretch or give in the system, however that gets put in...

I would caution Spidy in using the word "safer"... Safer all depends... To put less force on the overhead anchor point may or may not be safer...

Since it all depends: The bottom line is... it's good to have a variety of rigging lines available to use different situations and have a basic understanding of the mechanical principles to use a good rope. IE.. A dynamic line might be better than a static double braid in reducing force on the overhead anchor point...

 

[NeTree]

What's new about it?

 

[TheTreeSpyder]

Nuttin new bruddah, mountain rescue way ahead once again; jsut a chance to view forces from different seat, to remeber the patter to make better judgements about, when the view turns from the big picture, to a possibly more myopic event.

Certainly some surprising numbers, and a resounding definition that these forces we've tossed around here and there, perhaps deserve more attention, to make sure the technician is really stepping up to bat, and taking best shot for success and safety.

Here is an idea we came up with over at TB for dampening these forces, but not in such full realization for their 'impact' so to speak!

Glen-i'm surrrrrrrrrrrrrre enjoying the smaller file size with the MS-Paint strategy of initially saving in .Gif format; tons of differance!
Thanx,
-KC

 

[glens]

Originally posted by TheTreeSpyder
Glen-i'm surrrrrrrrrrrrrre enjoying the smaller file size with the MS-Paint strategy of initially saving in .Gif format; tons of differance!
Thanx,
-KC

Right tool for the right job, and all.  Would you use a 3/4" wire rope as a saw lanyard?  hahaha

Not only are the file sizes reduced by using GIF for these drawings, but the focus/clarity is better as well.&nbsp; If your program can save them as PNG it will result in further savings and PNG is a "free" (as in speech, not just as in beer) format to boot.&nbsp; Using the "convert" command line tool from the http://www.ImageMagick.org/ package to convert your most recent image to PNG we see:<blockquote>21351 kenslatest.gif
20848 kenslatest.gif.png</blockquote>Glen

 

[TheTreeSpyder]

Originally posted by murphy4trees
The point is: the force in shock loads can be huge and are best regulated by stretch or give in the system, however that gets put in...

I would caution Spidy in using the word "safer"... Safer all depends... To put less force on the overhead anchor point may or may not be safer...

Since it all depends: The bottom line is... it's good to have a variety of rigging lines available to use different situations and have a basic understanding of the mechanical principles to use a good rope. IE.. A dynamic line might be better than a static double braid in reducing force on the overhead anchor point...

The forces can be huge, and don't have to go to the supports.

Title was to get attention; safer as anything else is going to be dependant on:

What is the weakest point/per loading of the system chain-If this is the Pulley/support position, Hitchpoint (on more, not us tree guys delicate loads; but still a point in the system chain, of consideration to someone), Porty position etc. Then, at these times line consideration to these principals may be more applicable, than when line is weakest link. Of course upgrading line strength, without raising it's elasticity (by material design or length) places more peak loads on points in system connected by the line tension.

i think, that a dynamic line pre-stretched to the static range for the load, will give the less drop charachteristic and dampening of the peak forces that override the static range for the expected loads(?)

Whereby, a dynamic line can be stretched and made less dynamic/more direct shocking of loads in expected range, but stretch the elastcity on higher loading, to try to walk the best of both worlds(?)

Another weak link in a system, might be a new rope man on Porty on runs etc. the jerks to pulley/possibly climber position too, would be buffered.

So, increased safety is dependant on, if a stronger rope will make things worse, for the weaker part in the chain maybe the double loaded, leveraged position of climber etc.

Some of this is filtered out by every sliding, stretching, giving point under the high forces, but the rough pattrern remains in the chart diagram of the forces, jsut dropped down a notch, smoothed here and there. i just think the technician can drop the loading down a lot from their post as they set lines etc.

Be Safe,
:alien:

 

[rbtree]

Light lowering to me means a branch that is too heavy to grab and toss. this could be as light as 50 lb. With light loads, under 200 lb, I only occasionally use a pulley. If using a false crotch for loads tied above, that require some free fall before engaging, we'll break out the double braid and block. I like to lower off a natural crotch, often myself with one hand, which frees up a groundie. and with the friction there, it is easy to handle the weight. For these applications, you will never even come close to the rigging forces you've presented.
As far as dynamic line use goes, I can see its need in rare rare occasions, such as lowering off a dead spar, to further limit shock loading. But you have to have plenty of room to work with, as nylon lines have at least triple the stretch of polyester lines. Have maybe needed one a few times, but if I used one, it may have been some old Goldline years ago. This was the predecessor to modern kernmantle mt climb lines.

 

[TheTreeSpyder]

i always look to mountaineering/rescue; like they are ~50 years ahead on somethings; their lines are rated by tensile and elasticity; their names, even match the 2 forces of loading: Static and Dynamic. It is a variable, that they will alter there main tool (line) specifically to meet.

A dynamic force may have some static force in it, but static force has no dynamic force in it. An elastic line with a static load on it, pulls on the support the same as a static line, (it is only at motiion, that things change); so a static load does not use the dynamic properties, but a dynamic load can use the static properties; the ropes react exactly as in the forces that they are named for!

i think that for example, that a 5,000# tensile nylon will take a 7,000# shockload from a 3,000# load better, than a 6,000# static kernmantle (or some similar scenario).

Also, working without a pulley, natural crothes, reduces the amount of line to take shock greatly(more loading/shocking), forces more amplified through compounding with static line choice.

If you are dropping 200-600# class into line, and you want "Tiny" on the line as best portable pretightening and anchor; but his mitts want a larger diameter line, dang Cheaper ArboPlex, might be a better choice than StableBraid by this examination. The forces would be dampened by the greater elasticity in the system; due to the raw properties of the ArboPLex, and a higher percentage of the line tensile matching the loading. The line choice is a lil'shrewder, not jsut that easiset, just the strongest! In fact, only use the more expen$ive StableBraid when really need the clearance, and man really good on line; otherwise (at these loads) we might be really compromising ourselves by just jumping to strongest line blindly.

Sometimes i load a line over the center pillar of the tree, and then redirect it around to a carabiner on the closest limb etc. to load to lower to target as redirect of the supporty and brake force of top of tree to where i need it. Some of these branches for the redirect get kinda small,as i squeek by, sometimes even with a back up strategy etc. The smaller more elastic line helps i always thought....

i've switched lines when blocking out and catching on same spar as you stand on; i've felt the differance!

As RB says, not a choice that needs to be made every day for safety reasons, but certainly a chooseable option to alter when maximizing, especially where support loading matters more than clearance i think.

Or something like that
:alien:

 

[TheTreeSpyder]

There is something here, not needed all the times to be considered; but then sometimes yes. i think understanding the work in the system that the elasticity does as an essential element, especially as high as forces can go; is integral to seeing and respecting more clearly the functioning of the system elements seperately, then in orchestration. For greater safety, and efficiency both.

Or, something like that,
:alien:

From: Yale Cordage's Rope Engineering for Industry Page

Dynamic Loading Voids Normal Working Load:
Normal working loads are not applicable when rope is subject to significant dynamic loading. Instantaneous changes in load, up or down, in excess of 10% of the line’s rated working load constitutes hazardous shock load and would void normal working loads. Whenever a load is picked up, stopped, or swung there is an increased force due to dynamic loading. The more rapidly or suddenly such actions occur, the greater the increase will be. In extreme cases, the force put on the rope may be two, three or even more times the normal load involved. Examples could be picking up a tow on a slack line or using a rope to stop a falling object. Therefore, in all such applications such as towing lines, lifelines, safety lines, climbing ropes, etc., working loads as given DO NOT APPLY.

Users should be aware that dynamic effects are greater on a low elongation rope such as polyester than on a high elongation rope such as nylon, and greater on a shorter rope than on a longer one. The working load rations listed contain provision for very modest dynamic loads. This means, however, that when the working load has been used to select a rope, the load must be handled slowly and smoothly to minimize effect and avoid exceeding provision for them.

Example: A load of 3500 lbs. is being lowered using 5/8 diameter Double Esterlon which has a maximum recommended working load of 4000 lbs. With 15 feet of line payed out, the line is accidentally slipped, dropping the load one foot before arresting the fall.

Question: How much energy did the rope have to absorb, what was the maximum load on the rope, and has the rope been overloaded or damaged?

work done (Ft. Lbs.) - (weight)(length of fall) = 3500 Ft. Lbs.

rated maximum working energy absorption

capacity = (weight of the rope in use)(working energy absorption capacity for the type of line being used)

From the data page: Double Esterlon has a working energy absorption capacity of 544 ft. lbs. per pound of rope and a weight of 13.7 lbs. per 100 ft. or .137 lbs./ft.

rated maximum working energy absorption capacity of 16 ft. of 5/8

Double Esterlon =(16 ft.)(544 ft. lbs./lb.) = 1192 ft. lbs.

In this example, 2.19 lbs. of rope (16 ft. x .137 lbs./ft.) in use must absorb 3500 ft. lbs., or 3500 ÷ 2.19 lbs. equals 1596 ft. lbs./lb. or rope. Using the graph on page 8 (Double Esterlon), the right column indicates ft. lbs. of energy absorbed in capacity per pound of rope. Corresponding to 1596 ft. lbs./lb., you will find that the rope was loaded to approximately 48% of rated BS. From the data table, 48% of the 5/8" diameter Double Esterlon’s strength (16,000 lbs.) is 7680 lbs.

In this example the maximum working energy absorption capacity has been exceeded by nearly 3 times. The actual load on the rope reached 7680 lbs., or 3680 lbs. in excess of the maximum working load of 4000 lbs. Although difficult to quantify, some degree of the rope’s integrity has been compromised and prudent safety practice would call for downgrading or discarding the line. If in our example the load had fallen 7 feet instead of 1 foot, the rope would have broken.