Broken sling

[stig]

I'm bedridden with the flu, but the boys got a call from a logging operator, who's forwarder was stuck in an inaccessible place.
The service he usually uses to pull him out couldn't get in there, so could we please try and drag the big tirfor in there and see if we could get him unstuck.

Richard and Martin went in and set the 3½ ton Tirfor doubled up over a pulley ( see, this is why we have the big DMM pulley) and the GRCS doubled up over another pulley and managed to get him free.

But, while doing that, they also managed to break the spliced eye out of an almost new 44,500 pds breaking strength Samson Tenex sling!!!!

This while applying a constant pull of 15,000 pds.

No shock loading, no sharp bends, just a steady pull.

WTF?

I've written to Samson about it and expect to hear from them soon, but it would be cool if some of those here who know more about rigging than I do ( that would be most of themembers, probably!) could tell me what went wrong.

I'll have a hard time trusting my slings after this.

 

[sotc]

Where was the sling in the whole operation? Can you diagram your rigging?

 

[stig]

The sling was tied to the forwarder.
Big DMM block was in the eye of the sling with a dyneema ropeg going through it that was tied off to a tree in one end and to the winch in the other.

The weird thing to me was that it was the spliced eye that broke, not the knot tying the sling to the forwarder.

Is this explanation good enough, or shall I make a diagram?

 

[Al Smith]

If I read that correctly you have a 15,000 steady pull doubled twice ? That would be 60,000 .

The sling has eye splices which should be what maybe 60 percent more or less of the line strength or is that the rated load given for the sling ? If it was a single part hook up it sounds to me it exceeded the line strength .

 

[stig]

No, Al , the Tirfor pulls 3200 kilogrammes.
Double that and you have smewhere in the vicinity of 15ooopds.

Also, when a sling is rated to 44,500 pds, isn't that the breaking strength of the spliced eye?

Otherwise the rating is worthless, since it relies on the buyer having the knowledge to realize how much wa spliced eye reduces the total breaking strength with.

That sound decidedly un-American to me.

 

[Peter]

Two separate systems Al, so 15,000 pds doubled on the sling.

Safety factor on textiles is usually set at 10:1, so the safe working load of your sling would be 4450 pounds. Doubling the 15,000 pd line pull would be exerting a 30,000 pd force, so about 6.75 times the safe working load.

I wouldn't expect any sympathy from Samson, but I am puzzled as to why the splice failed rather than the knot. There should be very little strength loss in a locking brummel splice, especially compared to a knot.

Edit, just read new post above, so 3 times safe working load.

 

[pete mctree]

How was the sling configured? Was there a hard bend radius anywhere along the splice?

 

[stig]

No, it was holding the big DMM block and the other end was tied to the loader.

I would have expected it to break at the knot tying it to the forwarder, not at the eye splice holding the block.

I'll have to do a drawing of the set-up, I think.

Peter, if you double up a 3200 kg winch line, you get about 15000 pds on the block, I think?

 

[Peter]

I make it nearer 14,000, but I work better in kilos anyway.

It was a dead eye sling I presume? Was the eye large enough to comfortably fit on the block or did it spread the legs of the eye?

 

[stig]

Here is a drawing.

Sorry, but if I had artistic traits, do you think I'd have become a dumb logger!

 

[stig]

It is the largest sling they sell to arbos. It is made for that block, the block being just as overdone as the sling.

Which is why I'm somewhat freaked that it broke.

I mean, the sole reason we have slings and blocks that size ( besides us being gear freaks, of course!) is for that one job, where you just have to drop a top into your rigging in Murphy style and push the hell out off the envelope.

And then the biggest piece of artillery in my arsenal just breaks from pulling a forwarder up.

 

[pete mctree]

If the eye or then splice was not laterally loaded, then, I would query the bend radius @ the DMM block & how it sits in there. For maximum strength all the fibres need to be under tension.
It was a substantial pull that induced failure, however I , like most of us like to predict rigging failures & plan accordingly

 

[wiley_p]

1st. What does almost new mean? What other cycles had this sling gone thru? Also, in your drawing are the inside line angles you drew accurate? If that was a wider spread on those leg angles, you may have been exerting more than 15k of tension. Also that thimble on the big DMM is way too small to have favorable bend raidus on 1" stock. It failed at the eye cause thats where most of the heat was being developed.
I learned something last week at a meeting with real smart engineer types. When we exert a force on rope, individual fibers are permanently damaged. An easy way to see it is on those damaged fibers, a 100 pound load the 1st time can now have the stress of a 200 pound load the second time the rope is put into service. kind of a cycles to failure kind of deal.

 

[bonner1040]

3,200 kilo = 7054 lbs. doubled that equals around 14,000. The comment that was made about the angles does have an effect on the force exerted to the sling although I am not so sure of the exact factors.

I believe SWL is being interpreted incorrectly. For example a DMM large block has a MBS of 300kn which is 67,500 lbs; the SWL is 60kn or 13,500 lbs. Obviously a safety factor of 5:1 was used to determine the SWL.

This means that if you are handling a 13k lb item and it falls or in any way loads the rigging dynamically that the block will hold. This does not mean that the block should break under a load of 20,000 lbs.

I believe from Stigs description that the sling was under its MBS and as long as the load was slow and static it should not have broken.

Im no expert..

https://www.treestuff.com/store/catalog.asp?category_id=235&item=1718

I could not find a link to a sling as strong as the one in question.

 

[bonner1040]

1" Tenex SLING BREAKING STRENGTH: 44,500 LB. WLL: 4,450 LB.
http://wesspur.com/Slings/tenex-slings.html

 

[Peter]

A 5:1 safety factor is acceptable on hardware, but textiles should have a 10:1 safety factor for the reasons outlined by Wiley.

If you never exceed that SWL then the sling will last for many duty cycles, ie one loading and unloading. Exceeding the SWL even once will negate the cycles to failure and mean the sling could fail at a load well under the SWL due to damage to individual fibres.

My guess is that something got pinched or trapped at some point and damaged fibres in the eye, leading to premature failure.

I could be wrong though as I wasn't there.

 

[bonner1040]

My guess is that something got pinched or trapped at some point and damaged fibres in the eye, leading to premature failure.

That seems likely.

 

[lumberjack]

...Also, in your drawing are the inside line angles you drew accurate? If that was a wider spread on those leg angles, you may have been exerting more than 15k of tension...

Wouldn't a wider angle decrease the load on the sling? Unless I'm not thinking of some abstract setup, the most ma a single block can produce is 2:1 with the lines running parallel to each other. Spread the angle and the ma goes down.

 

[stig]

Dave, I was hoping you'd join in, since you have been doing breaking tests on stuff like this.

"almost new" means bought 3 years ago and sitting in a box ever since. This sling has not been used for anything, it is way bigger than what we normally use. In fact Richard has been getting on my ass for ordering 3 of them, which is kind of cute, considering he was the one who fell in love with the big DMM block.

As for the inside angles, I was in bed with a fever when this took place, so no idea.
I'll get on to that tomorrow.

If the swiwel on the DMM block is too small for the job in your opinion, aren't we all kind of screwed?

I mean, none of the blocks I own, have bigger swiwels.

 

[Peter]

The attachment point is too small, on all arborist rigging blocks.

The load on the pulley is twice the load on each leg of the pull rope, therefore the attachment sling used must be stronger and larger diameter than the pull line.

The pulley sheave is twice the diameter of the attachment point, yet the sling attachment point is supposed to work with the larger diameter line. All arborist blocks are designed back to front. The sling attachment point should be twice the diameter of the pulley sheave.

 

[Peter]

Just been chatting to Pete McTree.

How big is the eye on the sling?

Is it a brummel type splice or a straight bury?

1" diameter rope is over large for the block, meaning the rope is compressed inside the block and the fibres cannot lie straight under tension, creating uneven loading. The fibres under most tension would break, moving the load onto the next set of fibres, which would break in turn, until there were insufficient fibres to hold the load.

The action of the tirfor creates lots of peak loading moments, so its not really one continuous load but a series of duty cycles, this is accentuated by the static pull line.

A forestry type block would be better for this application, preferably attached by a chain rather than a textile sling.

 

[stig]

I'll take a picture of another of the slings tomorrow and post.

It didn't break the eye, but pulled it out, so fiber compression didn't really figure into the thing.

I'm learning a LOT from this.

Mainly that next time, I'll break out the hard rigging.
I just hate to carry that into the woods, but I guess that is one reason to have apprentices.

 

[bonner1040]

The dyneema held yes?

Sent from my C771 using Tapatalk

 

[flushcut]

Just read the above the splice pulled apart? A bad splice maybe?

 

[pete mctree]

If the splice pulled apart I would recon on the small bending ratio & fit when coupled with the DMM block. If only a percentage of the strands are tensioned at a time it would draw the splice apart piece by piece until the remaining had insufficient hold & therefore pulled apart. This scenario would be created by the slow loading of the winch as opposed to the dynamic impact it was designed for.

It will be interesting to see whether a brummel or a straight bury was used