How to figure the tension on a pole used for a zip line?

[oneelios]

I am looking all over to try and find out how to figure out the tension that would be on each of the poles used for a zip line I am trying to build in my back yard. I would like to build it so that it can support a 300lbs person. I am not very good with math so if someone would be willing to help me out with an equation that is for the simple minded it would be very helpful. Thanks!

 

[nvn]

oneelios: We need the (x, y, z) coordinates of all cable end points, the diameter and material of all cables, and the pole height, diameter, wall thickness, and material. Also state the initial sag (vertical distance at midpoint) in the transport cable.

 

[oneelios]

The cable is 5/16 I think-about the thickness of a standard pen. It will be about 90 feet long. I don't have it up yet because I need to know what tension will be on the poles before I decide how to build it. I am estimating roughly 10 feet high on one end and around 8 on the other. It will only be two poles. I would like as little sag as possible but won't know until I install it. Thanks for your response.

 

[nvn]

The poles will be under tremendous stress unless you have, e.g., guy wires. We need the coordinates of all guy wire end points. What is the cable material? And what pole material do you envision?

 

[oneelios]

The cable is actually metal-not sure what type though whether galvanized or not. I was considering a wood 8x8 as the main beam with a guy wire attached roughly at the point of the cable and then down opposite the cable direction into the base of another pole-possibly a fence gate post which is already in the ground and concreted or to the base of the mini light pole in our back yard. The other side may be attached to the base of an 8x8 that is part of our carport. I wanted to just wrap the cable around that 8x8 and call it good but I didnt want the carport to come down because I don't know the tension that would be on the pole so I was just going to use the base.

 

[Phrak]

And what keeps you from running your nether quarters into the fence post at ~20 mph?

 

[oneelios]

Well we will be hanging from a rope and there will be either a cable stop or some sort of tube slipped over the wire and a strong bungee cord hooked to that. I don't know for sure. I have been looking at many videos of what others have done. I just need to figure out the tension first that will be on the poles and then do the rest. I don't think it will be going 20 mph anyway-I could be wrong.

 

[nvn]

oneelios: What is the distance of the fence gate post base from the 10-ft-tall zip line pole base? What is the distance of the mini light pole base from the 10-ft-tall zip line pole base? What is the distance of the base of the carport 8 x 8 from the 8-ft-tall zip line pole base?

 

[Phrak]

oneelios: What is the distance of the fence gate post base from the 10-ft-tall zip line pole base? What is the distance of the mini light pole base from the 10-ft-tall zip line pole base? What is the distance of the base of the carport 8 x 8 from the 8-ft-tall zip line pole base?

The poles are under compression, rather than tension. For a pole, the failure mode would be through Euler buckling rather than via the compressive strength of the material.

The compressive force on a pole will depend upon the tension in the cable. The amount of tension depends upon how much sag you can tolerate without getting stuck in the middle of the cable. And this depends upon the friction in the pulley, the weight of the rider, length of the cable, distance and grade, and the elasticity of the cable.

Something occurs to me. You may not be aware, but the above assumes the cable is run over the over the top of the pole, then to an anchor, such as a tree or ground anchor, which would be the right way to do it. Don't anchor on top of a fence pole.

Additionally, anchors, poles and line should all lay in the same vertical plane.