porta wrap

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frodo

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what is the min. number of wraps you would need around the porta wrap for controlled decent of a 600 lb piece of wood.
 
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depends on how you are lowering it, crotch or block. Crotch rigged won't run as nice on a double wrap, but it is your safest bet. Double wrap block lowered would be no problem.
 
I dont think you cangive a definative answer on this. It depends on SO many things. Rope size, port-a-wrap size, distance it falls before the block/crotch, are you letting it run or does it need to be stoped instantly?

If your unsire take more wraps, this sounds like a troll question.
 
Very debatable question
are you sure it is 600 pounds? is your system capable of the forces generated by a piece that big? small medium or large port a wrap? do you need to control the piece before it hits the ground or can you let it run all the way to the ground? collar roping? bringing it easily onto tension?
2 - 4 wraps is my answer. we took pieces close to that last week with 1/2" stable braid, a big block, and 2 wraps on med port a wrap. Collar roped, we let them run to the ground. the rope was merely control for the fall and keeping them in place once they hit the ground.
If i needed to keep them off the ground to prevent damage i would definately have used a 5/8 or 3/4 rope and probably 3 wraps ( larger diameter rope more surface area = more friction. and more than likely smaller pieces.
kinda tough to judge without seeing the piece.
That should help but read on i am sure more posts will come.
 
thanks for the answers guys, i am trying to get an idea of what i can expect out of my porta wrap. there is no 600 lb piece to be lowered. i thought that weight would pretty much cover the spectrum of pieces i would be lowering:monkey: less weight less wraps.
 
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for arguments sake large steel porta wrap will be attached with 1'' stable braid ,3/4 to 1'' stable braid securing 1'' arborist rigging block, lowering lines will 1/2'', 5/8'',3/4'' depending on weight of piece
 
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I've easily lowered a 150 lb person on medium PAW with 3/4 wrap. Don't know the answer to your question, but if you know the forces involved in one particular case you can theoretically figure out how many wraps are needed for an entirely different case.

The relationship is exponential, and actually pretty interesting. As an example, if you can steadily lower a 100 lb load using 10 lbs of force and one wrap, taking another wrap will reduce the controlling force to just 1 lb. The actual force ratio you get will depend on the rope.

What is really interesting is the theoretical fact that diameter of the bollard you are using, whether PAW or a limb, as long as it is substantially larger than the rope, should make no difference! One wrap around a 3 in diameter bare limb should give the same force ratio as one wrap around a 10 in bare limb of the same wood!
 
quote: One wrap around a 3 in diameter bare limb should give the same force ratio as one wrap around a 10 in bare limb of the same wood!



i am not sure i know what you mean . if you were to take a 1 wrap around a 10 in. surface there would be more rope contacting the surface than with a 3 in. surface would'nt there? thus causing more friction and slowing things down more
 
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A greater length of the rope in contact with the wood, yes. But the actual force (think strangulation) pushing the rope into the wood is just that much less. The two things balance each other, and it is only the number of wraps that matter.
 
so are you saying that i could control the decent of a piece of wood with the same amount of pressure from my hands to the rope with 1 wrap around 10in or 3in. stem
 
so are you saying that i could control the decent of a piece of wood with the same amount of pressure from my hands to the rope with 1 wrap around 10in or 3in. stem

No, the contact surface in greater on 10 inches vs 3 inches, so there is more friction breaking.

Which is why every point of contact in the tree will add friction, thus breaking force. turning it into heat.

The old schooler can intuit how many wraps to give on a given size species of tree for a load. the advantage of a porty is that the breaking force is easily duplicated time after time. There is still an intuitive element, but a new ground man can pick it up easily after using the tool for a short period of time.
 
If your unsure take more wraps, this sounds like a troll question.[/QUOTE]

it hurts a hobbits feelings to be called a troll.

4 or 5 wraps seemed safe for large wood. just wanted to double check before taking less in a controlled environment. i will feel a bit more confident that no one will get there hands burned or lose control. thanks.
 
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No, the contact surface in greater on 10 inches vs 3 inches, so there is more friction breaking.

If contact surface were the only variable, this statement would be correct. But it isn't. The force pushing the rope against the surface is the other variable. The actual force squeezing the rope against the surface of the bollard at a given point is a consequence of two variables, the tension of the rope at that point and the radius of curvature at that point. The larger radius in the one case is compensated by proportionately smaller squeeze force. The result is the radius turns out not to matter--only the number of degrees of wrap.

I do not believe this to be true. It runs counter to my experience.

I can't argue with your experience, and I don't have enough experience myself to have an opinion on the matter. But I have a three general observations that might be relevant.

One, the equations governing friction are approximate. They don't have the precision and universality of, say, the law of conservation of momentum. Friction itself is poorly defined. When your rope is rubbing across the rough bark of a limb, and when the jagged surface of the bark is actually picking and grabbing the fibers of the rope (the velcro effect), do we count that as friction or something else? It certainly requires force to overcome, as does friction. The equations governing friction are idealized: they apply to the force required to slide two perfectly smooth surfaces across each other.

But there is no such thing as a smooth surface. At the molecular level, the surface is always rough.

Two, the equations state that the frictional force in a given situation is proportional to the normal force, that is, the force pushing the surfaces together. You can tell right away that this relation can only hold over a limited range of values, and outside that range, it will break down badly. (I can't apply a million tons to one square inch of surface and then measure the friction between the two objects--the objects will be destroyed.) But over a usefully wide range of values the equations hold pretty well, which is why they are printed in physics books and why a Port-a-Wrap works predictably over the range of loads that a person is likely to apply.

Three, the rope is deformed when it wraps around a bollard. Where the rope is under the greatest tension, nearest the moving load, it is flattened the most. This "footprint" of the rope is the area of contact, but the force is not uniform clear across this footprint. The normal force is much greater at the center and drops to zero at the very edge. At the other end of the wrap, nearest the controlling hand, the rope tension is much less, the flattening is less, and the footprint is much narrower, but the force distribution is still non-uniform. I suspect that when you combine the odd force distribution with the fact that both the rope and the wood have rough surfaces (meaning lots of velcro effect), you might expect the equations to break down a bit.

Still, if you did a careful experiment with two smoothly sanded limbs of different diameters, using a modest load of, say, 200 lbs or so, I'll bet you would find the controlling force nearly the same in both cases, close enough to conclude "this is a useful rule of thumb" rather than "this is not true."
 
4 or 5 wraps seemed safe for large wood. just wanted to double check before taking less in a controlled environment. i will feel a bit more confident that no one will get there hands burned or lose control. thanks.

4 or 5 wraps will utterly stop the load. Because of the exponential nature of the relationship, by the time you have 5 wraps on that puppy you can control a 1000 lb load with the force of a mosquito's wingbeat.
 
4 or 5 wraps will utterly stop the load. Because of the exponential nature of the relationship, by the time you have 5 wraps on that puppy you can control a 1000 lb load with the force of a mosquito's wingbeat.

I'll agree with that, 4-5 wraps is pretty much snubbing the load off.
 
Indeed. The practical limit seems to be about 3 wraps. I lowered half of a large American elm on 3 wraps. ( The tree had failed in a storm and fell on a house and I was rigging it off of the house.) Technically I didn't put half the trees weight on the porty-I'd snubbed off the top and brushed it out before lifting and lowering the butt-nonetheless I was dealing with weights I would never rig out under normal circumstances.
 
Now I use a bollard with 4" pipe and I drop huge pine tops all the time. Three wraps gives you superb control a little slow and four that thing ain't going anywhere.
Jared
 

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