Anybody here built a zip line?

[RobS]

The one's I have ridden (flown?) have had enough sag in them, with the starting end higher, so when you get to the designated spot you are going uphill again on the cable and naturally come to a stop. One particularly long one you would roll back and forth a few times before settling at the low spot. I don't think I'd want one where you could hit a tree!

Completely side note, I did another adventure race Saturday where there is normally a zip line. Instead, we rappelled off the face of a dam which ended in a 15 foot free fall to the water below. Was a blast!

 

[czechsonofagun]

I built one using stainless steel cable and some pullys from TSC. The riding element I welded together myself. One thing to work out is the tension of the cable - I ended up using one tightener in the line of cable an second one pulling it to the side forming a sort of Y on the bottom end of the line.

I test it myself every spring - 240 pounds make sure it is safe for the kids too

And tractor related part - there is an old garden tractor tire on the line to work as a stop/bumper.

 

[BruceWard]

Pulleys can be used to multiply the amount of weight used to tension the cable. A "block and tackle" is a good example.
Please see this for more info HowStuffWorks "How a Block and Tackle Works"

Using a block and tackle a 460 pound 55 gallon drum filled with water could be used to apply 920 or 1840 pounds of tension to the cable.

 

[PILOON]

I think he's saying if you put a cable between two post and pull it tight, then measure the tension on the cable. If it gets cold, the cable contracts, putting more tension on, as it tries to pull the posts together. Conversely, if it gets hot, the cable expands and the tension on each end diminishes.

Can you see that in a long cable run, that could cause the cable to sag in the middle quite a bit in hot weather and pull very tight in cold weather.

Now put a 500 lb trolley on that cable. In the winter, the trolley may get you over most objects very fine. In the summer, it may dip too low to get you over those objects. Lets hope the zip line doesn't run over a cactus patch!

By putting some weight on the cable past one end point, you can keep the cable at the same elevation all year.

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At same elevation and under same tension!

Pullies will actually multiply forces as in a come along or as on a winch using a pully.
One pulley does nothing but 2 doubles 3 tripples the forces or tension.

No pulley, ie fixed attachement at both ends could actually pull the anchorage out of the ground given the proper temp swings or at worse place so much tension on the cable as to even snap it, never mind the fact that at the center of the cable on a hot day you might just clear the ground and in the coldest you would be as high as your ancorage points.

A good sound tree as one anchorage would take care of tension problems as a tree is flexible and would act as a big spring or bungee.

 

[carpenter383]

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At same elevation and under same tension!

Pullies will actually multiply forces as in a come along or as on a winch using a pully.
One pulley does nothing but 2 doubles 3 tripples the forces or tension.

No pulley, ie fixed attachement at both ends could actually pull the anchorage out of the ground given the proper temp swings or at worse place so much tension on the cable as to even snap it, never mind the fact that at the center of the cable on a hot day you might just clear the ground and in the coldest you would be as high as your ancorage points.

A good sound tree as one anchorage would take care of tension problems as a tree is flexible and would act as a big spring or bungee.

A pulley all by itself dosen't multiply force. The pulling force is multiplied by splitting the counter force. Example: If I am winching a truck up a hill I can use a pulley (snatch block) to double the pulling force of the winch by running my winch cable through the snatch block and back to the winch (or a tree beside me would be safer) The snach block is then anchored with a chain to the truck. The hook at the end of the winch cable is now taking half the load and the winch itself is taking the other half.
If I were to just hook the end of the winch cable to the truck with the snatch block chained to a tree I wouldn't gain any pulling force.

 

[archerm3]

the tension in the string (cable) is always the same. i dont care HOW many pullies you put on the end. its still only (say) 500 lbs hanging off the end of the cable.

When done properly the end of the zip line cable is not the same as the cable holding the 500lb weight. The line with the weight, goes from the weight, to a pulley on a block, and back to an anchor, back to the pulley, back to the anchor, back to the pulley, etc. The zip line is connected to the block .

 

[RedRocker]

The zip line I rode in Ketchikan used two cables for each run.

 

[300UGUY]

Above info well presented,
but may I add:

Seasonal temperature changes will add stress (tension) as metal shrinks with cold!
For that reason ski tows always used weights at one end to keep tension constant.
I believe the safe way would be dead weight via pullies hung at one end.
With pullies you can 1/2 or 1/4 the needed weight.

Even bridges have expansion joints!

(not calculated) but even a 45 gal drum filled with concrete might be all you need (with pullies as multipliers)

I really don't see thermal expansion being a big factor. In 00 feet, I think it will be only a small amount, given a 80 degree temperature swing. Bridges have expansion joints, but the structure is a lot more complex than a "string" of steel. As I recall, the coefficient of expansion of steel is 60 millionths/inch/degree. I see steel wires all over the place, and they aren't sagging a whole bunch.

 

[300UGUY]

I really don't see thermal expansion being a big factor. In 00 feet, I think it will be only a small amount, given a 80 degree temperature swing. Bridges have expansion joints, but the structure is a lot more complex than a "string" of steel. As I recall, the coefficient of expansion of steel is 60 millionths/inch/degree. I see steel wires all over the place, and they aren't sagging a whole bunch.

Sorry, should have been "In 100 feet"!! duh!!!

 

[forgeblast]

The zip line I used has a tire stop towards the end so that you didnt go crashing into anything. The sag was also like a break slowly stopping us.
This was off of a 50' high telephone platform, you were harnessed in and you stepped off the platform and away you went.

I would look at Project adventure- Project Adventure - PA.org --they do the outdoor courses, training, and constuction
High and low course rope designs (google it)
A lot of boy scout camps have COPE courses now....challenging outdoor personal experience (cope). They will have climbing towers and high and low course events.