question about shock loading a rope and blocking

Re: question about shock loading a rope and block

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The results of dozens of scenarios were consistent: Any sling length whatsoever increased the ultimate shock load. The longer the sling the greater the shock. And the stretchier the sling the greater the shock.

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Thank you VERY MUCH for that.. I've seen even a great rigging "genius" show vids with a foot of more of slop in the sling, and have tried to caution people that keeping the block tied up as close to the cut and as tight as possible is the most important factor (that the climber controls) in reducing shock loads..

Yes its nice to have a good groundie that knows how to let a piece run... (poetry in motion and all)... and given that the ground man is going to do whatever he does, it is up to the climber to do whatever ever he can to throw the odds in his favor.. I always tie my sling before making the cut, which many dislike, but it allows me to get the sling snugged up tight and then get that cut as close as possible to the sling.

In negative blocking, my theory is that the final height of the block, when loaded, is more important a factor in determining force, than the weight of the piece.
 
Re: question about shock loading a rope and block

seems to me, that stretch of sling, stretch of rope, adjusted friction results in much less shock load.
While having a tight rope (pre-loaded) w/block close to cut surely eliminates shock...
also seems to me that any stretch eliminates shock, unless sling stretch allows log to fall farther at a somewhat fast rate... so, force allowance until max stretch is achieved would make the difference.
at what rate of force does the sling stretch vs. weight/fall of log would determine increase or decrease shock load.
so it seems to me.
 
Re: question about shock loading a rope and block

If the tree is the "ultimate" factor, and it is, as a failed tree is likely to kill, whereas failed rigging may do nothing more than dent up the lawn or crush some shrubs, then reducing shock loads on the rigging will also reduce loads on the tree. Thus the importance of the example in the yale cordage video, where the same shock load breaks a 20,000# low stretch line and will not break a 7,000# high stretch line.

I think there is a common misperception that shock loads only effect the rigging.. If that rigging is tied to the top of a tree, then the broken #20,000 line has put more than 20,000 lbs of force on the tree.

Bigger is generally faster and less fatiguing to the climber. I like to go as big as can be done safely, as long as the ground crew can process the pieces efficiently, which in most cases they can. Faster is safer IMO, less cutting, less rigging, less time in the drop zone, less fatigue, etc... As a climber its a beautiful thing to see the ground crew grab a 35' maple limb with the skid loader, untie the rope, then run it out to the chipper. I AM tying off the next cut as the chipper is sucking down the last of that monster limb and no one has touched the wood by hand.

When the ground crew can process big stuff that fast, it changes the way you approach the work. Its a whole new game.
Tuesday we did 2 crane jobs, first tree ( 42" ash) took 8 hours, and the second tree (60" sycamore), which proabably had 2x the mass of the ash took 4 hours.. the main difference was the size of the LZ... LZ for the first tree was in a tight driveway with overhead service wires etc.. 2nd tree had a 75'x 30' LZ in the closed street, with all utilities underground. Ground crew had time to rake the street between every pic.. It was a total pleasure.


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I'm wondering why the trend in rigging is bigger faster? For example, while a rope like dynasorb might allow you to take bigger pieces, why? I've always felt comfortable with the size pieces I take - lived in Cali and did the euc-man thing - learned allot, and have come to realize that the only advantage I see to a rope like dynasorb is that it could give me a bigger safety-factor with what I'm already comfortable doing. Why go bigger just because you can? (and who says you can - the tree is the "ultimate" factor here - not the rope and gear) Each rope construction has it's function - learning to use them appropriately in conjunction with all our other cool gear is the "art and science" of it all :)

peace,

mk

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Re: question about shock loading a rope and block

I just spent an enjoyable hour or so reading through the entire thread. One very common view seemed to be that a dynamic rigging line, or at least one substantially more so than Samson stable braid, would lead to lots of broken roofs, driveways with big holes punched in them, and so on. The term "rubber band" was used a number of times. What we need are some numbers.

Fortunately the Yale Web site has lovely stress-strain curves for many of its ropes, including Polydyne. Reading from the graph, it appears that Polydyne stretches 7% under a load of 20% of rated tensile strength. Plugging these numbers into my program I was able to compare 1/2-inch Polydyne against 1/2-inch Samson stable braid. The two ropes have virtually identical tensile strengths at 10,500 and 10,400 lbs., respectively, but Polydyne is much stretchier.

stable.jpg


polydyne.jpg


You can see from the screenshots that identical drops on the two ropes produce very different results. Note that there is a huge difference in shock load between the two, yet there is very little difference in the total length of the drop.
 
Re: question about shock loading a rope and block

right but daniel, we have seen your drop pieces that weigh more then 200lbs using true blue. true blue is 7,000lb breaking strength and your far exceeding the working load limit of 700lbs in some of your videos. stretch or not you still need to keep it within the working load limit.
 
Re: question about shock loading a rope and block

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Is that net drop or maximum drop before the rebound?

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Maximum drop. Specifically, I assume the rope is hitched around the piece exactly at the center of mass. In the examples the CM moves downward 8 feet, at which point the rope goes taut. After a bit of stretch all motion stops and the rope reaches maximum tension. The 8 feet added to the stretch is the total drop indicated in the screenshots.

I realize this is often far from a real-world scenario: the CM could be well above or below the hitch. The actual position of the CM relative to attachment hitch definitely will change the numbers, but it is really just a detail and it won't alter the overall behavior of the system.

There are other factors, too. The piece may have lots of small branches and leaves which break its fall. Maybe it folds over slowly on the hinge and ends up with less energy on that account. Or it falls in a big arc so that some of the kinetic energy is horizontal and contributes nothing to shock load.

The system I describe, given the assumption about the location of the CM (and the absence of a sling that can stretch) really describes a worst-case scenario as far as shock load is concerned.
 
Re: question about shock loading a rope and block

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right but daniel, we have seen your drop pieces that weigh more then 200lbs using true blue. true blue is 7,000lb breaking strength and your far exceeding the working load limit of 700lbs in some of your videos. stretch or not you still need to keep it within the working load limit.

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Give a brother a break Josh!
 
Re: question about shock loading a rope and block

One might question the accuracy of this black box program
into which the values are plugged --taking it as fact goes a
bit blind.

It seems surprising that such a slight difference (1 foot) can
provide for a near halving/doubling of impact force.

But realize part of the illusion here --drop distance:
10 vs 9 feet of TOTAL drop; but with 4' above block,
there's 8 feet of free fall before the rope comes into
tension.
.:. So, one sees a doubling/halving of stretch after tension;
that makes the output more palatable.

But one can also wonder about the results from a SUCCESSION
of drops, say, from ... chopping down a tree. Rockclimbing
tests for certification require only that the initial drop be
within specifications, and the subsequent ones --made at
some specified duration of rest-- simply continue until
the rope breaks, but they do NOT have to deliver the same
reduced impact (though, naturally, the less they reduce it
the sooner that rupture force is going to come).

Thanks for finding this.

- - - - - -

I, too, read the entire thread; can't say it was all so enjoyable,
as there was too much heat/passion and much less to think
about such as this program's results.

Might be helpful to give just a few more calculations with this,
at some other but possible values. Can the drop be shortened?
--lengthened? Another rope tried? And higher/lower blocking?
Maybe there are some conditions warranting special caution,
and others where one can see pretty clearly minimal risks.

*kN*
 
Re: question about shock loading a rope and block

@ D [ QUOTE ]

Bigger is generally faster and less fatiguing to the climber. I like to go as big as can be done safely, as long as the ground crew can process the pieces efficiently, which in most cases they can. Faster is safer IMO, less cutting, less rigging, less time in the drop zone, less fatigue, etc... As a climber its a beautiful thing to see the ground crew grab a 35' maple limb with the skid loader, untie the rope, then run it out to the chipper. I AM tying off the next cut as the chipper is sucking down the last of that monster limb and no one has touched the wood by hand.



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we are, it seems, on the opposite extreme. i have been fortunate to do (for a large, production oriented, international company - the crane/grapple-chipper/skid-steer/20" winch chipper/log-truck work - and it was challenging, exciting, hard, and satisfying. and big(ger)was king. but bigger wasn't always safer... and too often risk was pushed almost too far. so, in my quieter, very small - own company, bigger is not necessarily king, safe is king. this is why i say "when you know the limits of your gear, why test the tree?". we're talking shockloading rope and block, right?

peace,

mk
 
The Black Box

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One might question the accuracy of this black box program
into which the values are plugged --taking it as fact goes a
bit blind.

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I totally agree with this, and your other comments as well. Skepticism is the order of the day. I find myself in something of a dilemma: give all the details of the black box, which almost no one would read, or just report the results and try to make a point. I know if someone else had declared "I have calculated this, and here is the result," I would at least want to know how they did it even if the result seemed right.

Here then is a peak inside the black box. The math haters out there needn't worry--there isn't much math inside. There are two halves to the program: what the falling weight does and what the rope does, and the two halves have to agree. We assume all of the energy released by the falling weight is absorbed in some fashion by the rope. We separately calculate the released energy and the absorbed energy and compare them.

The energy generated by the falling load is easy to calculate; it is just the total distance the load has dropped times its weight. The total distance of the drop includes the amount the rope stretches once it goes taut, which we'll call S. Unfortunately we don't know the value of S.

On the rope side of things, the rope starts to stretch after it goes taut. The force needed to stretch it starts at zero and goes up in linear fashion until it has stretched the distance S. From the total length of rope available to stretch (34 feet in the example) and the characteristics of the rope published by the manufacturer, we can calculate calculate exactly how much force is required to stretch it distance S. Knowing this, we can also calculate how much energy it took to stretch the rope a distance S. Unfortunately we don't know the value of S.

We get to the correct answer by guessing. The program starts by guessing S = 1 foot. This will certainly be wrong. With S = 1, the total energy developed by the load is 900 ft-lb (9 feet times 100 lbs). Now the program calculates the energy absorbed by the rope if 34 feet of rope stretches 1 foot. The answer will either be bigger or smaller than 900, which means the first guess for S was either too big or too small.

The program then adjusts the guess for S, making it smaller or bigger as needed to bring the two calculations closer together. Everything is then recalculated...and so on, over and over again, until the energy developed by the falling load and the energy absorbed by the rope differ by no more than 0.01 ft-lb. At that point we accept the latest value of S as the correct value and show the results on the screen. In practice, it only takes about 4 or 5 passes through the loop to get to the final answer. The value 0.01 is completely arbitrary, but it gives us a final answer for the amount of stretch within about 0.001 inches of the mathematically truly correct value.
 
Re: question about shock loading a rope and block

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we are, it seems, on the opposite extreme. i have been fortunate to do (for a large, production oriented, international company - the crane/grapple-chipper/skid-steer/20" winch chipper/log-truck work - and it was challenging, exciting, hard, and satisfying. and big(ger)was king. but bigger wasn't always safer... and too often risk was pushed almost too far. so, in my quieter, very small - own company, bigger is not necessarily king, safe is king. this is why i say "when you know the limits of your gear, why test the tree?". we're talking shockloading rope and block, right?

peace,

mk

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My brother,
If bigger is the result of a culture where production is emphbasized at the expense of safety, that is a problem. I've heard reports about a few companies like that locally. They roll cranes, mess up all kinds of stuff, and men get hurt and killed.. The owners don't seem to mind..

Though "bigger" is often associated with that type of work culture, it is important to make a distinction between cause and correlation. Bigger does not necessarily make work unsafe, though it may be a result of unsafe work culture, that emphasizes production at the expense of safety.

In my world bigger is safer, though that may be somewhat of a rare scenario...
 
Re: The Black Box

I too was interested in Moray's results and recalculated the results using first principals, conservation of energy etc, rechecked the manufactures posted rope specs and had very similar results.

For those who are interested there are other calculation approaches contained in this report from the UK which is a great document reviewing rigging practice and current research. (large 16mb)

Evaluation of current rigging and dismantling practices used in arboriculture Prepared by Treevolution and Brudi & Partner TreeConsult for the Health and Safety Executive and the Forestry Commission 2008

Page 189 presents the equation used in the Rigging 1.0 software and another of the authors own derivation.

From a practical standpoint these kind of comparisons and results make me think about right sizing rope for a particular situation.

Norm's earlier observation of 6x loading is in line with Rigging 1.0 results and the use of stable braid. The same calculation using the polydyne provides a result of 3x. The most significant implications are for the rigging tip where we can have double the maximum rope load with a theoretical 6x reduction in loading using the polydyne.

It's easy enough to just grab the big rope and be sure it doesn't break. It's a little more difficult to right size the rope and rope characteristics which can significantly affect loading of the tree and ultimately job safety.
 
The UK Report

I downloaded the excellent UK report cited by grappleyarder and spent a few hours leafing through it. What a fascinating read! It is just full of interesting stuff and attempts to deal with the full complexity of arborist rigging scenarios.

In their many real-world tests where they measure everything (with high-speed cameras, load cells, strain gauges, etc., etc.), the actual measured shock load on a rigging rope is quite a bit less than would be predicted by the sort of worst-case analysis that I did. Much of the energy of the falling load does not actually end up in the rope, but in the block, in the sling, in the knots, in bending the stem, in breaking the hinge, in bending the root plate, etc. The rope is the easy part. I think my favorite part (and very useful) is where they go around breaking limbs of various sizes on several kinds of trees to develop some general rules on the strength of branches and branch unions.
 
Re: question about shock loading a rope and block

Here is a 92 second video of some work from last week. Big is in the eye of the beholder, so some might and some might not consider this big. I think it can be important when working from a bucket at full extension to cut a piece big enough to keep it balanced or butt heavy. I do it all the time.. set the lowering line with a pole saw and cut low enough to keep the piece somewhat balanced and let 'em swing out slow and easy, or keep the tips clear.

Couple nice falling cuts thrown in for some more crash and bang!

Hope you all are watching full screen

http://www.youtube.com/watch?v=0GQJkRcsoCo

and thanks for that link.

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we are, it seems, on the opposite extreme. i have been fortunate to do (for a large, production oriented, international company - the crane/grapple-chipper/skid-steer/20" winch chipper/log-truck work - and it was challenging, exciting, hard, and satisfying. and big(ger)was king. but bigger wasn't always safer... and too often risk was pushed almost too far. so, in my quieter, very small - own company, bigger is not necessarily king, safe is king. this is why i say "when you know the limits of your gear, why test the tree?". we're talking shockloading rope and block, right?

peace,

mk

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reducing shock loads

Thanks for posting this!
I think this is a brilliant post..

especially the conclusion:
It's easy enough to just grab the big rope and be sure it doesn't break. It's a little more difficult to right size the rope and rope characteristics which can significantly affect loading of the tree and ultimately job safety.

Well written! Hope someone is paying attention...


[ QUOTE ]
I too was interested in Moray's results and recalculated the results using first principals, conservation of energy etc, rechecked the manufactures posted rope specs and had very similar results.

For those who are interested there are other calculation approaches contained in this report from the UK which is a great document reviewing rigging practice and current research. (large 16mb)

Evaluation of current rigging and dismantling practices used in arboriculture Prepared by Treevolution and Brudi & Partner TreeConsult for the Health and Safety Executive and the Forestry Commission 2008

Page 189 presents the equation used in the Rigging 1.0 software and another of the authors own derivation.

From a practical standpoint these kind of comparisons and results make me think about right sizing rope for a particular situation.

Norm's earlier observation of 6x loading is in line with Rigging 1.0 results and the use of stable braid. The same calculation using the polydyne provides a result of 3x. The most significant implications are for the rigging tip where we can have double the maximum rope load with a theoretical 6x reduction in loading using the polydyne.

It's easy enough to just grab the big rope and be sure it doesn't break. It's a little more difficult to right size the rope and rope characteristics which can significantly affect loading of the tree and ultimately job safety.

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