Loopie sling break strength

B

born2climb

Location
Alabama
Can any of you rope/rigging experts explain to me why a loopie sling is rated twice as strong as a whoopie? I understand that it's doubled around the tree, but at the point of block attachment, there is ONE leg of sling running through the block...just like the whoopie. It seems to me that this would be the weakest point in the system, and that the rating should come from there. Wrapping an eye sling around the tree a dozen times doesn't increase its load rating, does it? The eye is the weakest part.

I bought up some tenex and made up some whoopies and eye slings. Now I'm considering loopies, but I never read of them in use, and the strength rating seems misleading.

Any thoughts?
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The rating is correct. It is true that ONE leg of the sling runs through the block, but for a given tension there, you have twice that much supporting any load. The block, in other words, can support a load equal to twice the tension.

Wrapping doesn't increase the load rating, but it does lend security to the loopie against sliding apart. If you make it to specs and use it the way it is meant to be used, it won't slide apart under load. It might not pull apart in a straight pull, either, but I would want to test that to be sure.
 
I understand that the loopie is doubled, but it's that single leg running around the block's anchor bushing that seems to be the weakest link. I'm sure the loopie is somewhat stronger than the whoopie simply because of its design, but I just can't see it being twice as strong. Maybe 60-80 percent stronger, but 100 percent stronger?
 
Hello,
Any eye, whether on a loopie or a whoopie, or a normal splice -- or any loop knot for that matter -- is twice as strong as the line it is made in. That's because there is twice as much material there. For that matter, the spliced portion of a rope is twice as strong as the standing part, for the same reason. Trouble is, all that load has to get funneled into a single thickness, for anything but a loopie. In the case of a knot, all that extra strength, and more, is lost as the energy deforms and compresses the material of the rope on the way to the standing part. Stress risers. In the case of a whoopie the deformation is less, just a change in surface helix angle for the most part, so it approaches 100% efficiency. And a good, tapered eyesplice transitions any deformations so gradually into the standing part that, as the extra mass is reduced in the taper, there's little or no stress riser.
In a loopie, though, there are two standing parts, so the strength is doubled for the entire length, with no more deformation than on a whoopie, and the effects of this deformation can be minimized by the positioning of the splice on the load.
All you need to maximize the efficiency of any of the above is to have adequate radius at both bearing points. An extremely tight radius, like say a knife edge, doesn't provide enough surface area for the force to transfer to the bearing point without damaging the fibers. As the radius increases, so do the number of fibers that bear on the bearing point. With a big enough radius there's no significant deformation or weakening.
Fair leads,
Brion Toss
 
I understand that there is twice the material at the attachment point. However, all the rigging weight is applied to the bearing surface of the block bushing. Hence, all the weight is applied to ONE thickness. In other words, though the loopie has two legs running around the tree vs. the whoopie's single leg, both have the block attached in an "eye"...the whoopie's spliced eye, and the loopie's "formed" eye. The difference in strength results from the portion of the sling running around the tree, correct? But both transfer all the rigging forces to the same critical point...where the bottom of the bushing (assumming vertical load) contacts the sling material. I know I'm not allowing for rigging variables such as line angle and so forth, but I'm trying to understand the strength ratings.

I intend to make up some loopies either way. I can see that they'd have a definite advantage in that they will adjust down to a smaller diameter, and more quickly as well.
 
I don't know if this will help or not, but from the Sherrill Tree catalog - love those catalogs, A whoopie sling made from 1/2" dia. Tenex has a listed WLL of 1,750 lbs.

A loopie sling made from 1/2" dia. Tenex has a listed WLL of 2,000 lbs.
 
Hi again,
Ah, I see the misunderstanding. Yes there is only one thickness of rope on the bearing surface, but it has the strength of two. To see this, imagine that instead of a block you have a bar, with a rope hanging from each end. Each rope is spliced, and is at or near 100% efficiency. You therefore have the strength of two ropes, right? The only difference with going around a radius with a single, continuous piece of rope is that the load from each half is distributed onto the entire radius, and not at a single attachment point. With the radius, there is no "critical point", unless, as I mentioned earlier, it is a knife blade.
For comparison, think of the axle of the sheave of the block that the rope might be on. The load does not bear only on the point of the axle that is opposite the load, but on either side of it, all the way down to where the load is tangent to the axle. The rope going over the sheave simply duplicates this, vectoring/depositing/siphoning its load onto the sheave (or tree, or whatever) as it goes around it.
Fair leads,
Brion Toss
 
Okay,
If I'm understanding you correctly, you're saying that the sling at its attachment point is as strong as TWO legs because it runs AROUND the bushing. I'm anot disagreeing outright, because I don't fully understand the dynamics. However, I seem to remember discussions regarding side-loading of a rope. In other words, picture a rope running horizontally from one tree to another, perfectly level. If the rope's rating is 10,000#, how much weight will it support, if the load is placed in the exact center? If I understand your example of the weight bearing around the radius of the bushing, then can I assume that the rope in my example will support 20,000# because it would be supported from EACH end, thereby TWO legs of the rope? Or, would it be more appropriate to assume that the one strand of rope will support the 10,000# that its rated for? I simply cannot grasp that a sling, be it a whoopie or a loopie, can be any stronger than the point of attachment, which I perceive to be a single run bearing on the bushing. I know two legs run TO the bushing, but effectively ONE leg supports all the weight at the bushing's bearing surface. I would appreciate a diagram if possible to help me visualize the proper dynamics. I am trying to absorb what you're saying...maybe I'm just slow...
 
Hi there,
I don't know how to make graphics appear, but let me try a puzzle. Let's say that you have a rope with a breaking strength of 10,100 pounds, and that you decide to pick up an object weighing only 10,000 pounds. So you've got that extra hundred pounds in reserve. You attach the rope to the object at one end, and then run the rope up and over a block, then down to a great big bucket, and suspend the bucket on the end of the rope just above the ground. You begin filling the bucket with water, and keep filling until there is exactly 10,000 pounds of water in the bucket, at which point your object floats delicately off the ground.
With me so far? We now have 10,000 pounds pulling down on the bucket side, and another 10,000 pounds pulling down on the object side, for a total of 20,000 pounds pulling on a continuous piece of rope that has a breaking strength of only slightly more than half that. Why doesn't the rope break? What happens at that block that seems to double the strength of the rope?
Fair leads,
Brion Toss
PS, I don't know why Sherrill lists the loopie strength at only slightly more than the whoopie. Could have to do with the relative security of the splices.
 
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...Fair leads,
Brion Toss
PS, I don't know why Sherrill lists the loopie strength at only slightly more than the whoopie. Could have to do with the relative security of the splices.

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Exactly, I'm uncertain about that myself, but it would be assuming a lot of risk to go beyond the listed WLL. It could even be a misprint, but I wouldn't want to assume that.

If we're making them up ourselves we may be assuming risk in a way(s) we don't realize.
 
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Okay,
If I'm understanding you correctly, you're saying that the sling at its attachment point is as strong as TWO legs because it runs AROUND the bushing. I'm anot disagreeing outright, because I don't fully understand the dynamics...

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I can't improve on what Brion has said, but imagine Brion's scenario with the block supporting two loads, each of 10,000 lbs. From the point of view of the rope, every inch of the rope sees the same 10,000-lb. tension, even the horizontal part directly over the block bushing. For this not to be true there would have to be some friction in the system between the two loads, but we are assuming a perfect pulley. If you agree with that, then there is nothing special about the horizontal part.

The WesSpur catalog shows the loopie sling for all 3 possible geometries having twice the strength of the corresponding whoopie.
 
born2climb,

I hope you're not feeling alone about this, forces in ropes and rigging can seem very puzzling and almost mystical.

The example you gave about the horizontal rope is different than the loopie/whoopie example because of the angle formed in the rope when the load is applied. The load and the angles of the rope determine the force the rope has to support. In the loopie/whoopie case the angles are essentially perpendicular to the load.

Suppose we take the example you gave and imagine a load of 100 lbs attached to the middle of the rope and the rope stretches/sags such that each side of the rope forms an angle 30° from horizontal. This forms a 'magic' angle of 120° between the two halves of rope and at that angle the force in the rope is equal to the weight of the load or in this case 100 lbs. At different angles you get different forces for the same load.

Now what if we could start moving the anchors the rope is tied to closer together so that finally, both sides of the rope are vertical - the anchors would have to be very close together, but the load in the rope due to the 100 lbs now drops to 50 lbs in each rope. But I don't think that's the puzzling thing being discussed here.

What we actually seem to be after is if you have a rope over a pulley (overhead) and you tie a 100 lbs to each of the rope ends, what is the force in the rope and why isn't the force in the rope at the top of the pulley sheave the sum of the two loads? After all, you've got 100 lbs pulling on one side and 100 lbs pulling on the other side. It would appear that somewhere the rope has to support 100 + 100 = 200, but the actual answer is 100, which makes us think that 100 + 100 = 100 and we know that can't be. And this is not easy to understand nor explain. To add to the confusion, there actually is 200 lbs applied to the anchor the pulley is tied to.

Suppose we eliminate the pulley and just look at a vertical rope with a 100 lb weight tied to one end. What force would we have to apply to the upper end of the rope to hold the 100 lb weight off the ground? 100 lbs. But, that in itself doesn't help much because we've got the same problem we had before - 100 lbs due to the weight at the lower end of the rope and 100 lbs pulling the opposite direction on the upper part of the rope which again appears to add up: 100 + 100 = 200 lbs in the rope. Unfortunately that isn't correct and it is very difficult to explain, at least for me it's difficult to explain.

Here's one way. In statics, the study of forces in equilibrium, we often take a 'section' through a member to analyze the force in the member. In this case the rope is the member and we'll use the vertical rope example we just discussed. The method is to take a section out of the member, and replace the section with equivalent force vectors. So let's say we cut a 3 inch section out of the rope and examine the lower portion of rope. Well on the lower end, we still have the 100 lb weight so what force vector must we have in the upper end of the rope? Well it has to balance out the weight and the only force that produces balance is 100 lbs. So we assign a vector, in this case simply a line with one end of the line attached to the rope, with the line pointing upward with an arrowhead on the end of it to indicate the direction of the force. We write 100 lbs by the arrowhead and we now have a complete force vector describing the direction and magnitude of the force at the end of the rope. Whew! So what force is supported at the end of the rope? We just wrote it down in the force vector - 100 lbs. That's the force the rope has to support.

Now let's go back to the pulley. It's the same procedure as before except we will section the rope right at the top most point on the sheave and we will now add a force vector to the end of the rope at that point. First the direction. Since the rope has wrapped 90° around the pulley, the force direction will be horizontal. What about the magnitude? It has to be equal to the load or 100 lbs in order for the system to be in balance. So what will the magnitude of the vector? 100 lbs. That is the force must the rope support at the top of the pulley.

I'm not sure that helps, but that's the statics way of looking at it.
 
More data,
I just spoke with Yale Rope, the developers of the loopie (yippie) sling. Their practice, in determining the WL of a loopie, is to double the strength of a single-part sling, and then multiply the result by .7. This is done to account for any weakening caused by the deformation in the spliced leg, and also because they just like to be conservative.
A whoopie sling would also, then, have a 30% reduction in WL for its splice, so a loopie would have twice the WL of a whoopie. Wesspur's catalog rating is right.
By the way, I always see to it that the spliced section of the loopie, right by where the running part exits, is on the bearing surface of either the load or the hoist. I put it there after milking all the slack out of the splice, so that the bury can't slide and loosen. Cheap insurance.
Fair leads,
Brion Toss
 
This is a very lucid explanation by Ron!

First I have to agree that a subject like this can seem obvious until you try to explain it, at which point you discover either you actually don't understand it or it is deeper and more complicated than you thought.

Let me try to supplement Ron's opus by going at it a little differently.

Imagine we have 100 lbs hanging on each side of a pulley. If I could magically lift the rope off the pulley long enough for you to insert your finger between the sheave and the rope, you would certify, when I released the rope, that the rope under tension was exerting a crushing force against your finger. But since the rope has the same tension everywhere it contacts the sheave, every inch of rope along the contact zone behaves like every other inch--each one of those inches exerts a crushing force against the sheave aimed at the axle. The magnitudes of these forces are all the same, but their directions are different. This is how the tensioned rope applies force to the pulley. It is almost ironic that the mysterious segment of rope right at the top of the sheave, the "horizontal" part, exerts its force straight downward, and contributes more than any other equal-length segment to the final result.

If we can add up all these force vectors we will end up with the actual load experienced by the pulley axle. This is most easily accomplished with a little calculus, which I won't go into here, but the vector sum of all those little arrows is one big arrow pointing straight down whose magnitude is 2 X the rope tension.

There is nothing special pulleys and ropes as far as the physics goes. Anytime you have a number of forces impinging on an object, all the individual forces add up vector-wise to a single force vector. If you have 7 strong men trying to push a car out of the mud, they can't all fit at one end of the car where their respective force vectors would be aligned perfectly with the desired motion of the car. That doesn't mean 4 men sit on the sidelines. They arrange themselves along the sides of the car because they know instinctively that even the misaligned force of a somewhat sideways force still contributes to the result. In our pulley problem, all the forces are misaligned except the one at the very top, but the vector sum is straight down.
 
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...By the way, I always see to it that the spliced section of the loopie, right by where the running part exits, is on the bearing surface of either the load or the hoist. I put it there after milking all the slack out of the splice, so that the bury can't slide and loosen. Cheap insurance.
Fair leads,
Brion Toss

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I have often wondered about this, and it is actually high on my list of things I want to test. I have always considered the loopie to be an eye splice in which the entire load is applied to the buried core. The bight in the sleave where the load is applied, and the fact (in tree work) that the whole thing is also wrapped around a tree, keeps the core from pulling out.

In the attached photo the dotted line shows the "eye" that isn't there. The point marked "T" is where the tapered end of the bury would be. The arrow points to the throat of the splice, and this is the where the cover would normally have no tension, and therefore would not be squeezing the core. Heavy stitching at this point, as called for by the manufacturers, essentially performs the same role as a clamp or a taut cover would--it allows for significant tension in the cover in the vicinity of the arrow, and therefore significant friction against the core. Since the cover tension is a max just beyond "T", and zero just to the left of the arrow, anything we can do to increase core squeeze at the arrow would seem desirable. There is no need for action in the vicinity of "T" because the cover is automatically under full tension there anyway. I conclude (but, as I say, I intend to test this) that the load point and bight should be at the arrow to make up for the missing stitching. Is this what you meant, or do you apply the load at "T"?
 

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you guy are still getting to deep. go shallow. both create a " chocking effect on the load bearing surface. in one "choke" the woopie sling has a part of line going through the bight. in the loopie the "choke" has two parts going through the bight. the bight is the cause of the reduced efficiency in the line. the rope will fail were the greatest amount of rope efficiency is lost. being that a cut in the line, a knot in the rope, a snag, or bight. most knots in the line cause a 50% efficiency drop, most hitches cause a 20% reduction in efficiency. a "chocker" effect on the line causes a 40% reduction in efficiency.
 
Okay, sorry I asked. It seems I've opened a can of worms. I was simply questioning the comparitive strengths of the loopie and whoopie. Thanks for all the info. It does help me to understand it a little better. I still cannot understand how a line running over a block, with each leg supporting 100# does not have 200# effectively applied to it at the top, as this is the amount of force applied to the blocks anchor point.

However, I am satisfied that you got the math right. I have no way of proving one way or the other. I just wanted to make sure that I was staying within the proper working load limits of the loopie. As I said before, I perceive the loopie's eye and the whoopie's eye as being the weakest point, as that's where the block's sheave is going to impart all its force.

Thanks for all the input, and by all means, if anyone has anything to add, keep the discussion alive.
 
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...By the way, I always see to it that the spliced section of the loopie, right by where the running part exits, is on the bearing surface of either the load or the hoist. I put it there after milking all the slack out of the splice, so that the bury can't slide and loosen. Cheap insurance.
Fair leads,
Brion Toss

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I have often wondered about this, and it is actually high on my list of things I want to test. I have always considered the loopie to be an eye splice in which the entire load is applied to the buried core. The bight in the sleave where the load is applied, and the fact (in tree work) that the whole thing is also wrapped around a tree, keeps the core from pulling out.

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Actually, no. The load is applied to both "core" and "cover", because they are, in uncovered ropes, one and the same. This is true for whoopies, loopies, and conventional eyes. And neither the bight (by which I assume you mean a choked attachment) nor the tree keep the core from pulling out,per se. That is,it is easy to achieve maximum splice efficiency without the presence of either, though they can provide backup security.

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In the attached photo the dotted line shows the "eye" that isn't there. The point marked "T" is where the tapered end of the bury would be. The arrow points to the throat of the splice, and this is the where the cover would normally have no tension, and therefore would not be squeezing the core. Heavy stitching at this point, as called for by the manufacturers, essentially performs the same role as a clamp or a taut cover would--it allows for significant tension in the cover in the vicinity of the arrow, and therefore significant friction against the core.
Since the cover tension is a max just beyond "T", and zero just to the left of the arrow, anything we can do to increase core squeeze at the arrow would seem desirable. There is no need for action in the vicinity of "T" because the cover is automatically under full tension there anyway. I conclude (but, as I say, I intend to test this) that the load point and bight should be at the arrow to make up for the missing stitching. Is this what you meant, or do you apply the load at "T"?

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Oy. First, the absence of the eye at "T" changes how the load is applied so much as to make "T" meaningless. In a conventional splice or a whoopie, half the load is applied by what is more or less an appendix in a loopie. But in all cases, the splice works because of friction generated along the entire length of the splice. Stitching in a conventional splice has no bearing on ultimate splice efficiency; it is there to make sure that the splice doesn't slip before enough load comes on to clamp the outer part around the inner. It serves no other purpose. In a whoopie or loopie, just as with friction hitches, or for that matter any knot or hitch, this anti-slip function must be provided by the user, who dresses the knot or splice, to make sure it is in a condition that will take the load without slipping. That is why, when I position the bearing point on the splice, I do so at the point that can most readily slide. Of course, if the bearing surface is big enough, I'll put the whole splice on it.
Fair leads,
Brion Toss
 
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