Help with understanding WLL

[Jonchitect]

Hiya Folks,
I was hoping someone could help me understand what the WLL is on a negative blocking kit when all the components have different WLL ratings.

For instance the WesSpur standard rigging kit has a 5/8” loopie sling, WLL 1692 lbs, with a block that has a 2000 lbs WLL. A 1/2” rigging line wll 1400 lbs, and a port of wrap with a WLL 2000 lbs.

Does this mean that my WLL for the whole system is only as much as the lowest WLL? Also. If I understand it right then the maximum weight of a log I could negative block is 140lbs. Do I understand this correctly. Any clarification would be greatly appreciated, thanks for taking the time.

 

[Benjo75]

Your rigging rope should be your weakest link. I don't see how you could destroy a port a wrap. The sling and block would be next in breaking order depending on what your using. Keep in mind that a percentage of rigging points (the limb you're rigging from) will fail before your gear does. Especially before the sling and block. The WLL is the largest load a device is intended to handle. That's usually 10 percent of the break strength. Breaking strength is where a device fails. Whatever your lowest rating is should break first. That is age and wear dependent though. You can have a brand new 1/2 inch 7,000lb rope and your block tied to an old faded wore out 9/16 sling rated new at 10,000 and the sling will probably break first. Especially since it's seeing almost double the load that the rope is seeing.

Looks like 1,400 lbs is as high as you need to go with that particular rope. Negative rigging can greatly start to multiply rigging forces. This is the page from Wesspurs 2020 catalog explaining it.

 

[theTreeSpyder]

WLL is an honor rating, some with less honor rate theirs higher, not real hard set standard.
>>real pro manufactures to to rate conservatively to their honor of quality, and long term lack of chance legal litigation.
Rate your chain of support by weakest link>> rope>> but can also be elastic function needed by this same position.
.
As stay 'safer', using larger capacity line to same work (like varying sizes/strengths of ArborPlex of same materials and design)
>>Drops elastic dampening response in trade for more strength/capacity headroom.
>>As does doubling line to 2/1 support on pulley to load, more capacity, but less elasticity against same impact hit
Tho, enough strength is good, too much strength against hit impact can be too stiff/strong per loading to give dampening response return.
>>strength headroom gives more strength capacity, but impeding on capacity is what reveals elasticity.
Goldilocks of mid range, not extremes of pitfalls can be best in dynamic hit!

 

[murphy4trees]

The 1,400 lb WLL of 1/2" line you mentions sounds like it's 20% of the 7,000 lb tensile strength. Your 140 lb pieces sound like 10% of 20%. That's not the way the math is supposed to work, You figure the weight of the piece, plus the added force on the line from a shock load. Then keep that within some percentage of the tensile strength of the line. I always use 20% for rigging. I've heard 10% as an industry standard, but not clear where that number came from.

And the math in Blair's formula is extremely conservative and does not play out accurately in the real world, because it assumes a dead stop. With good rope (stretchy true blue) and a good rope man to control deceleration, you can keep the force on the rope to about 3x the weight.

I negative rig up to 800 lbs on true blue without much concern, but I replace my ropes pretty often.

 

[Jonchitect]

WLL is an honor rating, some with less honor rate theirs higher, not real hard set standard.
>>real pro manufactures to to rate conservatively to their honor of quality, and long term lack of chance legal litigation.
Rate your chain of support by weakest link>> rope>> but can also be elastic function needed by this same position.
.
As stay 'safer', using larger capacity line to same work (like varying sizes/strengths of ArborPlex of same materials and design)
>>Drops elastic dampening response in trade for more strength/capacity headroom.
>>As does doubling line to 2/1 support on pulley to load, more capacity, but less elasticity against same impact hit
Tho, enough strength is good, too much strength against hit impact can be too stiff/strong per loading to give dampening response return.
>>strength headroom gives more strength capacity, but impeding on capacity is what reveals elasticity.
Goldilocks of mid range, not extremes of pitfalls can be best in dynamic hit!

Thank you Sir,
When you talked about doubling line to 2/1support on pulley to load more capacity. Could you clarify? Is that double whipping? Or like a fishing pole system but with only 2 pulleys?

 

[Jonchitect]

Your rigging rope should be your weakest link. I don't see how you could destroy a port a wrap. The sling and block would be next in breaking order depending on what your using. Keep in mind that a percentage of rigging points (the limb you're rigging from) will fail before your gear does. Especially before the sling and block. The WLL is the largest load a device is intended to handle. That's usually 10 percent of the break strength. Breaking strength is where a device fails. Whatever your lowest rating is should break first. That is age and wear dependent though. You can have a brand new 1/2 inch 7,000lb rope and your block tied to an old faded wore out 9/16 sling rated new at 10,000 and the sling will probably break first. Especially since it's seeing almost double the load that the rope is seeing.

Looks like 1,400 lbs is as high as you need to go with that particular rope. Negative rigging can greatly start to multiply rigging forces. This is the page from Wesspurs 2020 catalog explaining it.

Hi Benjo, thank you for clarifying my questions. Could I trouble you to explain why the sling sees 2x the force that the rope does?
I thought each end of the rope caught the weight of the log attached to the sling?

 

[Jonchitect]

The 1,400 lb WLL of 1/2" line you mentions sounds like it's 20% of the 7,000 lb tensile strength. Your 140 lb pieces sound like 10% of 20%. That's not the way the math is supposed to work, You figure the weight of the piece, plus the added force on the line from a shock load. Then keep that within some percentage of the tensile strength of the line. I always use 20% for rigging. I've heard 10% as an industry standard, but not clear where that number came from.

And the math in Blair's formula is extremely conservative and does not play out accurately in the real world, because it assumes a dead stop. With good rope (stretchy true blue) and a good rope man to control deceleration, you can keep the force on the rope to about 3x the weight.

I negative rig up to 800 lbs on true blue without much concern, but I replace my ropes pretty often.

Hi Murphy,
Thanks for taking the time to respond to my question.

 

[murphy4trees]

Happy to help a brother!

 

[Jonchitect]

The 1,400 lb WLL of 1/2" line you mentions sounds like it's 20% of the 7,000 lb tensile strength. Your 140 lb pieces sound like 10% of 20%. That's not the way the math is supposed to work, You figure the weight of the piece, plus the added force on the line from a shock load. Then keep that within some percentage of the tensile strength of the line. I always use 20% for rigging. I've heard 10% as an industry standard, but not clear where that number came from.

And the math in Blair's formula is extremely conservative and does not play out accurately in the real world, because it assumes a dead stop. With good rope (stretchy true blue) and a good rope man to control deceleration, you can keep the force on the rope to about 3x the weight.

I negative rig up to 800 lbs on true blue without much concern, but I replace my ropes pretty often.

Hi Murphy, thanks again. I also wanted to ask what Blair’s formula is. Is it (length +1 x Weight = force)?

 

[Benjo75]

.

 

[theTreeSpyder]

View as Load and ballast to float Load w/o friction are both 100# pulls against pulley, tied(not too load) or held.
>> if vertical pull w/o friction to pulley, doesn't match load , load moves per imbalance of forces.
.
w/o pulley on load/ redirect on support anchor only:
If have a 5k rope and replace with 10k matching materials and construction of rope.
>>hit will impede on rope capacity less, to give less elastic dampening response/ more raw hit.
Similarly, take 5k rope and put thru pulley on load
>> terminate 1 end on upper support that also has redirect pulley (one not on load) to ground.
2:1 on load, capacity/head room less encroached on, is stiffer / less elastic against hit
>>in trade for increased power.
Extra rope in system gives more rubber band generally, but not in this loading pattern that does not extend rigged length to same ballast point.
.
So, a monster bull rope can ruin day in impact etc. on wrong load.
>> wood flexing, rope stretching etc. are reliefs, can soften hits like steam relief valve.
>> leaving less contained forces behind, after peak forces allowed escape.
.
Rope is the softer, wearable, replaceable device, smoothing things out like fanbelt.

 

[Tree09]

When we are using different things to rig with, they all have different breaking ratings. So, how could we ever use them safely to do different tasks? The accepted method to add engineering tolerances and acceptable life cycles is to use a wll. Not all wll are the same, it depends what the use is and what the material is. For example, a wll of 3:1 is commonly used for guy lines, 5:1 is most static rigging, 10:1 is used for life support, duty cycle work, and cordage ropes because of their stretch. Although rated 5:1 a wire rope stretches too in use, each strand rubbing and grinding on each other as the load comes on, shearing, flattening, bending and grinding past every sheave, work hardening each strand til they break or corrode away. But cordage ropes stretch even more usually, and are subject to even more abrasion and rubbing because of it.

All hardware and rigging also is further derated on the rigging and the piece interact, all knots, splices, connecting pieces, pulleys, etc. Most knots remove 40 to 70 percent of the acceptable wll of a rope, and any damaged strands will reduce it even more. Even using a rated sling, the simple action of choking the limb derates a sling by 40 percent from its straight line pull rating regardless of its construction. And that's assuming you are rigging properly with the properly sized shackle to ensure sufficient d radius for the material and to do the rubbing. In industrial rigging, you actually have to add up the efficiency of each pulley to the actual line pull of the winch, taking in account for both line stretch and rope weight and sling angle, to come up with that situation's exact wll for that one application.

Any dynamic loading will greatly lower the wll, because not only is the load coming on and off very quickly, making the abrasion even worse, it often overloads a certain spot, which is forced to take a far higher load. Negative blocking, while necessary at times, is literally the absolute worst thing you can do to a rope. Its a full on factor 2 fall, and even with the world's best ground guy on the other end will show localized loads that far exceed the weight of the piece. The rope isn't perfectly elastic, so even time between cycles becomes a factor. The end will see increased wear from all these factors as well.

Obviously, the weakest link in a rigging chain is the limiting factor, and it should be your rope. Sling angle and configuration both come into play on the rigging block. Also to be considered would be any side loaded hardware, such as a carb or a snap.

 

[Jonchitect]

When we are using different things to rig with, they all have different breaking ratings. So, how could we ever use them safely to do different tasks? The accepted method to add engineering tolerances and acceptable life cycles is to use a wll. Not all wll are the same, it depends what the use is and what the material is. For example, a wll of 3:1 is commonly used for guy lines, 5:1 is most static rigging, 10:1 is used for life support, duty cycle work, and cordage ropes because of their stretch. Although rated 5:1 a wire rope stretches too in use, each strand rubbing and grinding on each other as the load comes on, shearing, flattening, bending and grinding past every sheave, work hardening each strand til they break or corrode away. But cordage ropes stretch even more usually, and are subject to even more abrasion and rubbing because of it.

All hardware and rigging also is further derated on the rigging and the piece interact, all knots, splices, connecting pieces, pulleys, etc. Most knots remove 40 to 70 percent of the acceptable wll of a rope, and any damaged strands will reduce it even more. Even using a rated sling, the simple action of choking the limb derates a sling by 40 percent from its straight line pull rating regardless of its construction. And that's assuming you are rigging properly with the properly sized shackle to ensure sufficient d radius for the material and to do the rubbing. In industrial rigging, you actually have to add up the efficiency of each pulley to the actual line pull of the winch, taking in account for both line stretch and rope weight and sling angle, to come up with that situation's exact wll for that one application.

Any dynamic loading will greatly lower the wll, because not only is the load coming on and off very quickly, making the abrasion even worse, it often overloads a certain spot, which is forced to take a far higher load. Negative blocking, while necessary at times, is literally the absolute worst thing you can do to a rope. Its a full on factor 2 fall, and even with the world's best ground guy on the other end will show localized loads that far exceed the weight of the piece. The rope isn't perfectly elastic, so even time between cycles becomes a factor. The end will see increased wear from all these factors as well.

Obviously, the weakest link in a rigging chain is the limiting factor, and it should be your rope. Sling angle and configuration both come into play on the rigging block. Also to be considered would be any side loaded hardware, such as a carb or a snap.

Thanks for adding this to consider

 

[murphy4trees]

Its a full on factor 2 fall,

are you sure about that?

 

[Tree09]

While if you are using blocks, since there's more rope in the system so it's not a true factor 2 fall, even though I think I've seen studies that show that the end takes far more abuse and stretch than the rest of the rope. Using natural crotch and the new friction devices such as the rings and such up top move everything closer to a 2 factor fall. While you would be hard pressed to actually make something a factor 2 fall while doing tree work, someone who isn't familiar with rigging should learn about fall factors so they can account for it during rigging, and maybe choose alternatives that are more static with a lower fall factor if possible.

 

[CurSedVoyce]

One of the reasons to pertension rope on negative blocking.

 

[murphy4trees]

except for everyone else here let that misstatement about negative rigging being a fall factor of 2 slide. WHY?

There is NO WAY that rigging from an overhead block could EVER be a fall factor of 2. Total nonsense and no one here makes a peep. Once again I have to straighten you all out about such simple concepts.

Then there is the OP suggesting that the max weight of a log that can be lowered on 1/2" is 140 lbs... Here's 1,100 lbs rigged n true blue

 

[CurSedVoyce]

If your pic drops 6 feet to a sudden stop, that would be a FF2.

 

[murphy4trees]

Fall factor is not about the actual measurement of the length of fall. It's about a relationship between the distance fallen to the length of the line in the system. The fact that rock climbers and the rescue folks are more concerned about fall factor that distance fallen should tell you something about how important stretch in the line is.

 

[Tree09]

And no one said that line stretch is a bad thing. If you are natty crotching, and it's snubbed off, it's a fall factor of 2. I natty crotch a bunch, and run lines that can take the abuse, either arborplex or treemaster 3 stand. Having said that, you need to know the limits of that, and fall factor eventually becomes an issue using natural crotch rigging. If you are using blocks, it's never gonna be a full on factor 2 fall, as i mentioned in my second post, which was only made because you didn't like me mentioning fall factors without a complete dissertation to someone asking basic questions about wll. I have read articles before where they did testing on buil lines during blocking operations, and they found increased wear on the portion past the block. I'm too lazy to find them, but i remember them, obviously from some isa or similar publication.

You can't watch your video on here, and so i didn't watch it. You have the option to watch videos on websites blocked. I seldom watch videos on here anyways, on my phone and all. The op was asking how much should he derate the rigging to account for shock loading, a very valid and important question because it's very significant. He was thinking 10 to 1, which is too extreme obviously. If he's climbing vs working from a bucket is another huge consideration, you can do flat out stupid shit in a bucket safely, where if you are climbing you are stuck there.

Rather than getting complicated as hell going through math, we can get a pretty good idea of what rope manufacturers think about it by how much climbing ropes have to be tested and rated for, which is 5k pounds min. Knowing that life support is 10:1, we know that the max load they suspect the rope to see is 500 pounds. Assuming even a 250 pound climber, that's roughly 2x what the guy weighs. So if we are using blocks, and our weakest gear (from the op) is the loopie sling at 1700 (because the rope is going through a pulley it's seeing 2x the weight). 1700/2 is 850, so going much more than 400 is going to be banging up on the wll of the gear. Getting a proper loopie sling that is rated more than the rope would be a brilliant idea, and would make the rope the weakest part, meaning you can go more like 700 before limiting out your gear. I personally don't like going big as possible, because tearing gear up isn't how i do stuff, so personally I'm going less than that.

 

[theTreeSpyder]

250# climber rescuing 250# + saws-n-Gear =540# x10swl =5400 is one way i have always looked at that.
>>i believe in other disciplines say 5k, but 5400 in arbo for wearables vs. 5k for durables is how i learned it. (original arboplex 5400 i believe std. was written to support )
>>tree work ropes harsher conditions, with less rope scrutiny (mountain climber mite very kindly offer to throw you of ridge if step on their line and crush sand into jacket)
>>has been suggested that the last 400# strength above 5k attributes to this extra wear anticipated, but then there is the chainsaws etc. more particular to tree worx
Slings should be stronger as stated, as usually , (unless on load) are a redirect not a termination (or simple pass thru).
.
In our rough numbers in tree work, i think Fall Factor talk mostly focuses on minimizing the fall, more than length of rope taking hit.
>>tho this scenario does have pulley, many other tree rigs have friction at that point reducing effectiveness of elastic dampening of rest of line.
More than pure numbers, these things are about what points to respect, how much in the pattern asserted.
>>numbers reveal language of working patterns below the thin veneer/skin that eye sees
>> i think we focus more on the risk than relief here to maintain drama, to keep safe.
>>out of lab and computer, in real time OJTSOSMF(On the Job Training Sink Or Swim MoFo) these are all guesses, risks kept high numbers and releifs not counted on as much type safety.
Fall Factor can be induced as well as reduced by this drop vs. rigidity against drop.
Mountain /rescue lines lots more elasticity, usually more length and usually controlled slip/running against lighter loads.
.
Any slip, give, elasticity etc. is like relief valve on pressure cooker >> relieving highest steam pressure to keep kettle together.

 

[murphy4trees]

If you are natty crotching, and it's snubbed off, it's a fall factor of 2. ...
If you are using blocks, it's never gonna be a full on factor 2 fall,
The op was asking how much should he derate the rigging to account for shock loading, a very valid and important question because it's very significant. He was thinking 10 to 1, which is too extreme obviously. If he's climbing vs working from a bucket is another huge consideration, you can do flat out stupid shit in a bucket safely, where if you are climbing you are stuck there.
I personally don't like going big as possible, because tearing gear up isn't how i do stuff, so personally I'm going less than that.

the falling side of the rope is still going to stretch NC or not.

And Blair's formula is:
force = weight of piece + (weight of piece x number of feet in fall)

so 500 lb piece falling 5 feet would be 500 + (5 x 500) = 3000

Distance fallen is from the center of gravity of the piece at the start to CoG at rest below

That is a very inaccurate and misleading formula because it accounts for no stretch in the rope and no run in the system, as shown in the above video.

The OP hasn't made it clear where his idea that the piece should weigh no more than 140 lbs comes from.

And yes, the working load limit of the entire system is only as great as the WLL of the weakest link.

 

[Tree09]

Yeah I've never seen that formula, it doesn't account for anything and I'm not sure how anyone came up with that. A falling body is constantly accelerating (at 32 feet per second per second), so any force calculation is far more complicated and exponential than that (it's called impulse in physics). It also isn't taking into account the time of impact (stretch) of the rope or any running. It might work decently enough, especially if someone is conservative with it, but I've never seen that before. Where does that come from?

 

[DMc]

... Where does that come from?

Kids nowadays. So you have not heard of Don Blair?
I'm pretty sure he was just trying to develop a simple to use, guide for the mathematically challenged tree guy.

 

[Tree09]

I have heard of him, but i don't have his books and haven't memorized his works. So there is the rough formula, cool.