loading of anchor trees and line stretch

[IllinoisJim]

I am working on setting up a small static highline and pulley to winch wood up steep hills by lifting them off the ground. I did a quick set up just to see how things were going to work. Used about 1/2 of final loads and got a lot more deflection of the main line than I expected. So much that it was unusable even with the lower loads. I anchored one end of about a 120' span about 10 feet up in a tree that was about 10" dia. I anchored the other end about 15 feet up in a 15" tree. 5/16 Amsteel. I hand tensioned the line without any pulleys or other mechanical advantage. When loaded with about 100 - 150 lbs the line sagged clear to the ground. I thought it might just be initial tightening of the rope strands (amsteel stretch is speced at 1% I believe but I thought it might stretch more from an initial, loose condition) so retightened the attach points and repeated this loading, unloading, tightening sequence several times. I now think the sag was due at least in part to the flex of the trees so I plan to run "guy lines" from the attach point to the base of a nearby tree. Does this sound reasonable? I don't see any other sources of stretch or " give" but may be overlooking something.

 

[clearance]

Hand won't tighten 120' of anything.

 

[Rftreeman]

guy the trees then use a mechanical device to tension the line, should work..

 

[treesquirrel]

Hand won't tighten 120' of anything.

Yup,

You are gonna have to set up a MA to get anywhere near the tightness you will need for any significant loads.

 

[TimberMcPherson]

I am working on setting up a small static highline and pulley to winch wood up steep hills by lifting them off the ground. I did a quick set up just to see how things were going to work. Used about 1/2 of final loads and got a lot more deflection of the main line than I expected. So much that it was unusable even with the lower loads. I anchored one end of about a 120' span about 10 feet up in a tree that was about 10" dia. I anchored the other end about 15 feet up in a 15" tree. 5/16 Amsteel. I hand tensioned the line without any pulleys or other mechanical advantage. When loaded with about 100 - 150 lbs the line sagged clear to the ground. I thought it might just be initial tightening of the rope strands (amsteel stretch is speced at 1% I believe but I thought it might stretch more from an initial, loose condition) so retightened the attach points and repeated this loading, unloading, tightening sequence several times. I now think the sag was due at least in part to the flex of the trees so I plan to run "guy lines" from the attach point to the base of a nearby tree. Does this sound reasonable? I don't see any other sources of stretch or " give" but may be overlooking something.

Tensioning skylines isnt easy, they need ALOT of tension. The higher the point the better. Dont tie into the tree, run the rope through a pulley at the desired height and the use a tree or stump behind to tension it at ground level using a Z drag or attach into a winch.
I havent worked with amsteel but I dont think you can use prussics or acender type devices on amsteel so you may have to use alpine butterfly knots if your doing the Z drag thing which could take some careful measuring.

 

[AT sawyer]

A highline tensioned to it's maximum safe working load limit will be subjected to overtensioning once you run a load up it. The point of maximum overtension will be midway between your spar trees. I used a TU-28 Griphoist to tension a 250' 3/8" steel cable highline and had to keep my loads under 500lb for the WLL safety factor. Even under full tension, the wire rope sagged significantly from the load weight. Highline tensioning is serious business and there's not much info available online. The spar trees just hold the blocks and elevate your wire; you have to anchor both ends of the wire to something solid or you could flex/pull the spar trees over. Hand tighten? Forget about it.

Attached pic has about 300lbs of locust logs riding the wire. Lewis winch did the lifting; double blocks kept the load straight, as we had a narrow line window.

 

[IllinoisJim]

AT Sawyer,

Thanks! I plan to do close to 250' spans with about 250 lbs max load - about half what you had. I agree that you can pull the wire too tight. The more sag the less tension in the main line to support the load. I found some info on line but a lot of it has to do with really big rigging (1"+ steel cables!) like used out west. But it can be scaled or just do the math - it's not too bad. I figured the best way to control the load was to estimate the load from weight tables and make sure I have at least the needed sag to keep the tension within limits. Mainline tension is the only real limit. The pull line tension is less than the weight of the load.

Would like to hear more about your setup and see more pics. Did you slack and tension the main line to lift the loads? You say a lewis winch did the lifting - do you mean pulling the load up the cable once it was off the ground?

Jim

 

[AT sawyer]

We slacked the line after each load. We put the blocks (Skookum A6s) as high in the spar trees as we could get and picked spar trees that had other big trees close behind them and in line with our wire so we could anchor to the base. The Lewis winch had an easy time of it, as the loads were never heavy, however, we put smaller redirectional blocks on the Lewis wire so it would align with the highline. Otherwise, as you get to the end, the Lewis is pulling down on your highline adding more tension to it.

This was my first experience with a Lewis winch, so I ran a test of all the hardware in my back yard with some landscape timbers just to prove it would work. I made a YouTube video while I was doing it in case other guys from my trail club were interested in the setup. I apologize in advance for the simplicity of it, but the lessons learned paid off when we dragged all that gear up the mountain for the actual log haul.

http://www.youtube.com/user/PATCsawyer#p/a/u/2/nulOLxehBpk