do we need an eye-splices???

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A lashing hitch (cow hitch and girth hitch) are shown to have "35% of possible strength" which means that they have a strength loss of 65%.

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Okay. Theoretically, say you've got a chord that'll do 5,000 lb and you form one of those hitches with it. Since there are two legs of the (now doubled) chord immediately carrying the load, you're looking at 10k possible capacity. 35% of that would be 3,500 lb, which is 70% of the original 5,000 lb, right? That would be a 30% (original) strength loss for better results than the bowline.

Of course, this is assuming the hitch isn't formed in a sloppy manner such that the bend/bite would be subjected to high friction/temperature due to (quick?) relative movement as the load is absorbed.

I'm just flying a kite with this...

Glen
 
[ QUOTE ]
[ QUOTE ]
A lashing hitch (cow hitch and girth hitch) are shown to have "35% of possible strength" which means that they have a strength loss of 65%.

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... you've got a chord ...you form one of those hitches with it. Since there are two legs of the (now doubled) chord immediately carrying the load, ...That would be a 30% (original) strength loss for better results than the bowline.

Glen

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Although the legs share the load I don't think they share the load equally. I haven't seen any data on this, but empirically it seems that the leg that holds the block bears more of the load than the leg that forms the half hitch. Even if the lags did share the load equally, it still seems that the bight/bend of the cow hitch is the weak point. That part of the knot has rope on rope friction and has the sharpest bend. If this is the weak point of the hitch it doesn't matter how much stronger the splice or bowline or doubled chords are, because the hitch will break at the bight/bend.

I don't know what reference(s) Blair used for his breaking strengths, so I don't know how accurate and/or up to date they are. I think this thread asks a good question and it would be interesting to compare Blair's strength ratings to other sources.

Maybe RescueMan could offer some insight.

Mahk
 
We just need to tie a sling up in a tree, put a HUGE sling below it at the base of the tree with a BAP (big ass pulley), run a rope from the top sling, through the pulley and tie to the back of a HAT (huge ass truck). Have the truck go until the sling breaks (rate everything so that the sling is what breaks) then see where it breaks.

Any takers? Send me the rope and I'll splice up the slings for testing. I'm sure we can find someone to tie the bowlines!

love
nick
 
i think strength wise a tension less hitch on non small diameter mount is best; but, that wouldn't resist pulling at all the angles right.

i think you have a round turn or reaarnge it's path into a clove as 2 basic mounts. For a 3rd variation reverse clove half way thru for cow, but still about as many turns as round turn. Has a sharper bend(weaker tensile)as it makes it's hook grab around loaded tail, but resists pulling in more directions,than clove and roundturn hitch bases. Also, cow prevents walking compared to clove, roundturn if loaded right until, you take 1 leg out of lock of cow, to form a muenter that gives walking capability.

i think Cow's strength (beyond blocking pull up and over unlike round turn and clove bases) would be in the teepee angle formed at mounting by size of mount and tension in cow; both factors making up a given bend angle as product, that reduces the efficiency of the line. i think that only legs of line attached to the block or brake device share the load. So i don't think that would count the overhand lock finish. But could concievieably count a long eye as 2 legs of line taking inital loading, that buries it's jointing behind friction buffer from main load pull (totallly clear of bight that chokes around eye pushing on jointing). i think strenghts will be differnet pulling parallel or perpendicular to mount. A roundsling in choke will be about 80% of tensile (vertical sling tensile); a cow is like a 80% roundsling choke cut open to bear load on 1 leg.

/forum/images/graemlins/propeller.gif
 
Ha, it looked funny to me at the time, but the browser's "built-in" spell-checker didn't flag it, so I guess I'd meant "chord" in the old Websters meaning of "cord".

The only data I can readily find is in http://www.yalecordage.com/html/pdf/Arborist.pdf (590547 bytes; link still good at this time), where on page 9 are listed ratings for their Eye/Eye, Endless Slings, and Adjustable Slings, at various diameters, used in Vertical, Choker, and Basket configurations. In the several (at least one of each product configuration) usage types I checked, they have the Choker rating at 80% of Vertical, and 40% of Basket.

I'll attach the single page as an 11270 byte PDF file.

Assuming the typical usage of an arborist block slinged (slung?) to a stem, in essence we have a choker configuration at the cow/timber hitch. 40% of the basket configuration (which we're approaching) rating, less a bit more for less-than-ideal conditions (friction/side-loading), or maybe 35% of the basket rating, sounds pretty much the same as the previously-referenced work (and I'd bet that's how they'd figured it).

Naturally, the end of the cord attached to the block would be in the "vertical" configuration, so, yes, the hitch to the stem (with its marginal extra loss in capacity) would then be the weak point in the system. But not by much, if the eye attached to the block is spliced and not formed with a bowline (unless the bowline were far enough away such that it was part of the wrap on the far side of the stem, hahaha!).

Did Frans and Gerry use anything like this in their Volvo drop?

Glen
 

Attachments

i think of a linear is the tensile strength of the loop or eye to eye; from there it is standard to calculate a choker as 80% of that tensile, basket as 200%. Which makes a choker 40% of basket, a basket 250% of choker.

i think a cow, is more like half of a choker; a single leg choke, like you cut the sling open while in choke position; bringing us back down to ~40% tensile of a line/tensile or roundsling we just cut open. It had 80% strength with both legs going to block/brake device, but now only has 1 leg going to the block, though laced the same.

The basket only hits such high numbers, if it's legs hang straight, and unleveraged. The choker's strength loss can too be higher than the optimum rating of 80% tensile. If choked down real tight, not letting a more relaxed bend, but tighter one in the neck of the choker, will leverage the device more, for lowering usable tensile strength % i think.

Samson Ropes tech once told me that 2x strength for basket was more of a convention; that only 2 seperate lines would have 200% tensile (100% strengh terminations assumed). He said that they calculate conserveatively at 2 legs of same line as 165% of tensile, not 200% for in house calculations; because that is the way they tested out on some stuff, on the fancy machines.

A sling wears differnt positions on day to day loading, an eye-to-eye wears 1.
 
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Although the legs share the load I don't think they share the load equally

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Absolutely no way do they share the load equally. There's too much friction. As you get further into the sling, the load lessens. The only way to share it equally would be to create a basket situation by havind an eye in each end and hook them both to the block. Even then I bet there won't be equal sharing.
 
i think a cow made with a long eye, whereby there were 2 legs of self equalizing loop going through the bight that restricts the loaded leg(s)to block or brake, is more akin to the strength of a choke loop @ 80% tensile, than a cow at 35-40%. The eye gives 2 legs like the choke, rather than 1 leg with an eye on the end like a cow. The 2 leg choke loop is rated @ 2x the strength of the single leg choke cow. A cow is like the choke loop is cut open, for 1 leg of support with block/brake device on end. The long/buried eye walks the line inbetween, but still offers 2 legs of support before the choke, and decreasing amopunt of force the farther around mount it is hidden towards back. If not satisfied, bury the jointing of the eye deeper and take round turn before choking around eye to device.

i also think any roundsling loop or dual leg system allows for some failing or slipping of one leg, as the more static leg takes the load and load adjusts; and therefore are superior to single leg systems; in this self checking feature generally (when not engaging slip systems, inverting the principle).

We now continue with (y)our normal programming!
/forum/images/graemlins/propeller.gif
 
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Using {the}reference (Rigging For Removal) an eye splice has a strength loss of 10% and the bowline has a strength loss of 35-40% (p. 11). ...A lashing hitch (cow hitch and girth hitch) are shown to have "35% of possible strength" which means that they have a strength loss of 65%. /forum/images/graemlins/crazy.gif

Using this reference, the cow hitch is much weaker than the bowline.

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These numbers didn't seem right to me and I think I found what the confusion is. In the knots section of the table Blair shows strength loss compared to the rated tensile strength of the rope in a straight pull, but for the hitches he shows strength loss compared to the highest possible sling configuration (basket hitch) which is actually double that of the tensile strength of the sling.

Blair shows a table that gives " % strength loss" for a splice and for various knots. It's not stated, but I would guess that this means strength loss compared to the rated tensile strength of a rope.

Blair then shows line drawings of various hitches formed with an eye and eye webbing sling and labels them as a "...% of possible strength." The first hitch that he shows is a basket hitch, formed with both eyes of the sling on a hook. This is labeled as "100% of possible strength".

The next hitch that he shows is a vertical hitch, formed with one eye on the hook and the other hanging with a shackle in it. This is labeled as "50% of possible strength".

He then shows a choker hitch ("40% of possible strength") with the sling choked on a log.

Finally he shows "Lashing Hitch: 35% of possible strength." But, he is comparing this hitch to the basket hitch, which he says has "100% of possible strength". The sling is doubled in the basket hitch, so it will actually bear approximately twice (200%) the rated capacity of the sling. The vertical hitch is shown as "50% of possible strength", but this is in relation to the basket hitch. The vertical hitch will actually hold 100% of the rated capacity of the sling. The lashing hitch shows "35% of possible strength", but if this is also in relation to the basket hitch, then it is actually 70% of the rated capacity of the sling (or a 30% strength loss). Note too that the sling that is shown for the lashing hitch is a dead eye rope sling.


Getting back to Marcin's original question, according to the Rigging for Removal Workbook, an eye splice causes a 10% strength loss, a bowline causes a 35-40% strength loss and a lashing (cow or timber) hitch causes a 30% strength loss. The lashing hitch is (slightly) stronger than a bowline and if a block is attached to a sling with a bowline, the bowline is the weak point. Replacing the bowline with a splice would make the lashing hitch itself the weak point and would make the whole system stronger.



Mahk



another long post
 
Getting back to Marcin's original question...."Do we need an eye splice?"

No, we don't. We could use bigger rigging slings, tie the knots, then deal with the clumsy knots while up in the tree. You can get around not having the splice.

But why would you? It is simply easier to tie the cow/stilson hitch when you are using a spliced sling. I suggest that after you consider time fumbling in the tree, in the long run the spliced sling ends up cheaper that the tied bowline.

love
nick
 
Pretty sharp Mark!

i still kinda think that 2 legs of line supporting loadings to Brake or Block is better than one. Whether that is 2 seperate Cow Hitches (pulling evenly), legs of a round sling, similarily legs of long bowline/eye that hid jointing behind buffering friction from load (and from equal and opposite restriction to loading that secures), 2 eyes of 'Line' to Block placed for support legs- then thru bight/loop to choke to mount etc.

Favoring the RoundSling choker or Long Splice or "Long Eyed Cow" for this, for the self equalizing on a loop that can be ridden in position to all directions. Rather than 2 independant legs at different tensions to different directions of pull; that 2 seperate Cows or Eye-to-Eye with both eyes on device, then choked.

Theorizing, rope bridge from device to mount as weakest link to upgrade in Cow. Long Eye kinda puts device in basket, rather than linear for that part of the mounting to upgrade weakest link. Place jointing of splice or bowline away from primary loading, buffered by friction. Keep forces lower in choke by not flattening teepee on mount too. The choke restriction in all of the strategies has 2 legs, why not this bridge?

When using a Timber Hitch (short on line etc.); it is harder to secure if you don't have enough line to spiral back closer to the original pull, as system locking force. Halfway around a stump on a Timber Hitch 'twirl' and running out of line; isn't maximum pinching force to secure, so seems like a good place for bowline etc. buffered from direct forces of Cow Hitch.

Favoring the Bowline (DBY) for adjustability to place jointing in specific position, and not wearing the same point in a spliced eye.
 
Holy crap, I was just wondering when this thread would die, then when I looked at it, it had 666 views! Weird. I think the guy that started the thread is gone away, haven't heard from him for awhile. /forum/images/graemlins/shocked.gif
 
Nick, I don't think he was saying it was better...I think his point was that the cow hitch was the weakest part of the whole rigging system, so using a bowline didn't weaken that rigging (or to say it from the other side, that the spliced eye didn't make the total rigging stronger). I think we most all agree that the spliced eye is handier, but he may have a point, especially if the bowline is tied with a long loop, as described by Tree Spyder.

I can see why this seems incompehensible to you, the splice guru /forum/images/graemlins/wink.gif.
 
Burnham, I think you're on to me. I guess my previous post was my way of saying, "yes, maybe the bowline is adequate." But why would you opt for that over a splice.

I basically other than educational value, this conversation is moot. /forum/images/graemlins/pbj.gif

I wanted to make sure that people weren't actually saying the splice was inferior. I got the sense that some were ready to make that claim.

You called me on it, though! I do see things through the splicers glasses!

love
nick
 
In Long Eye Cow strategy:
If you want to get the jointing to ~center rear on a 6' diameter tree on Monday, same position on an 8' diameter on Tues, and same position on 10' on Ths.; a fixed position of a splice might not be best. With so much line 'tied' up in getting to center rear on 10' diameter (or could be less), it might be considered a waste to have that amouunt of line resource, in a splice permanently etc.; infringing on flexability of length of line use.

Guess we would fairly try a short eye splice on Cow(single leg of device support going into cow), vs. an 8' long bowline eye, that presented a loop of 2 legs of support to device in Cow lacing, and bowline knot/jointing only got loading after 7'+ of friction around trunk.
 
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Nick, I don't think he was saying it was better...I think his point was that the cow hitch was the weakest part of the whole rigging system, so using a bowline didn't weaken that rigging (or to say it from the other side, that the spliced eye didn't make the total rigging stronger). I think we most all agree that the spliced eye is handier, but he may have a point, especially if the bowline is tied with a long loop, as described by Tree Spyder.

I can see why this seems incompehensible to you, the splice guru /forum/images/graemlins/wink.gif.

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I agree that he was saying that the cow hitch was the weakest part of the system but I think he was wrong. From my post above:

"... according to the Rigging for Removal Workbook, an eye splice causes a 10% strength loss, a bowline causes a 35-40% strength loss and a lashing (cow or timber) hitch causes a 30% strength loss. The lashing hitch is (slightly) stronger than a bowline and if a block is attached to a sling with a bowline, the bowline is the weak point. Replacing the bowline with a splice would make the lashing hitch itself the weak point and would make the whole system stronger."


The bowline is weaker than a splice and weaker than a lashing hitch. But, I think Spydey is right that the bowline will bear less force if it is tied with a long loop, because the friction of the loop on the tree reduces some of the force on the knot itself. The same can be said for a splice, however, so it is a moot point. You could tie a bowline with a long loop, or make a splice with a similar length loop. The splice would still be stronger and easier to use.

Splice Guru may be biased towards splices, but I think he's right.

Mahk
 
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