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Zip line sag. Keep tightening or no?
June 2, 2012 4:58 PM   Subscribe

Zip line filter: I'm building a zip line on our property. The issue I'm having is cable sag. Help?

The line is anchored at each end by heavy chain around very robust trees, with the cable hooked to the chains. The cable is 3/8" (14400 breaking point) 7-19. The run is 310 feet from start to end with a drop of about 6-8 feet (hard to tell because the ground drops also).

I am able to tighten the cable until it is near taut (NOT piano wire taut); then I test with a "crash test dummy" hanging from the trolley and weighing about 240 pounds (the weight of the heaviest person that will use it). After 1 or 2 runs with the crash test dummy, the cable is noticeably no longer taut. Per the instructions I got from the kit supplier, who said that new cable stretches and must be re-tightened, I have re-tightened it - probably 6-8 times. Each time it's near-taut after tightening, and then noticeably not so taut after a few runs.

The supplier has now told me that the reason the cable keeps stretching is because I'm tightening it too much. I'm unclear as to what this means. He seems to be saying I should stop tightening it. I'd appreciate feedback from zip line users out there about this issue. Is there a point where the stretching stops? Should I stop re-tightening the cable and deal with the 'sag' by raising the cable at both ends? Thanks as always.
posted by charris5005 to Home & Garden (13 answers total) 2 users marked this as a favorite
 
Is it possible to increase the drop?

I think that by tauting the line, you simply stretch it to become longer (unwinding the twist on the cable).
posted by Rabarberofficer at 5:02 PM on June 2, 2012


Taut cable should not sag. Your slippage must be coming from where it is attached to the chains. How are you hooking the cable to the chains? A good zip line is never taut but always has some sag built in to act as a brake.
posted by Xurando at 5:12 PM on June 2, 2012


6-8 ft drop over that length really isn't enough. You need to start higher because your cable needs to sag some as Xurando says. For a 300 foot run I think you need to start at least 15 feet up, preferably more. Our zipline was about 80 feet and started some 15 ft from the ground, ending at 8 ft, and that worked pretty well.
posted by anadem at 5:35 PM on June 2, 2012


You're never going to be able to get a taut line, ever. Google 'catenary sag'. In short, it requires an infinite amount of tension on the line to be taut with a weight on it. Since the line can't handle that, sounds like the line's stretching until it can. Allowing some slack in the line will put less stress on it.
posted by suedehead at 6:04 PM on June 2, 2012 [4 favorites]


What suedehead says. Cable has weight.
posted by rhizome at 6:29 PM on June 2, 2012


Not an engineer, but you might want to test with a crash test dummy that weighs more the actual weight of the people who will be using it.
posted by ActingTheGoat at 7:13 PM on June 2, 2012 [5 favorites]


My (awesome) dad solved this, back in the day, by putting a turnbuckle on the line. Steel stretches, there's no way to avoid it. A turnbuckle makes it easy to periodically tighten the line, which you do have to do from time to time. If you have to do it after every other run though, I would suggest that maybe you need to get a thicker cable which will stand up better to the rigors of ziplining.
posted by Scientist at 8:23 PM on June 2, 2012


Are you using a come-a-long? I have never seen a zip line in person, but I assumed they used a come-a-long to get proper tautness.
posted by sanka at 8:49 PM on June 2, 2012


If you do the trigonometry on a working zipline with a 300' run over 8' of drop, you'll find that to support 250lbs at the 3/4 point, you'll be generating somewhere on the order of 7000+ lbs of load on the cable. This gives you less than a safety factor of 2.

This is *insane* for system designed to support human life, and steel cables stretch significantly (and get weaker quickly) when stressed to that close to their failure point.

Personally, I'd want to load the cable to more like 1/5 to 1/10 of it's failing load before I felt comfortable with my family or friends using it. Also, under safe loads, you'll find that it stretches much less.
posted by ccoryell at 10:07 PM on June 2, 2012


Geometry is working against you here. Or maybe you're trying to work against geometry.

Try anchoring one end of your 300 foot zip line to the towbar of a truck instead of a tree. Put the parking brakes on. Go grab the middle of the zip line and walk it sideways. If the truck doesn't get dragged backwards for at least a few feet before you even break a sweat, I will be very very surprised.

The geometric form you need to be thinking about is a pair of straight members with a hinge point between them, almost but not quite in line with one another. A little bit of sideways force at the hinge point produces a disproportionately huge end-to-end force.

Think about locking pliers. Those things get their absolutely insane grip by using the very same geometry your zip line sets up, only inside the pliers mechanism the forces are compressions rather than tensions. And you know how, with vice-grip pliers, you have to fiddle with the adjustment screw for a while before you can get them to provide that last little bit of irresistible death-grip without needing to hurt your hand squeezing? What you're doing with the adjustment screw is getting the two compression bars to be exactly in line right where the jaws need to exert ultimate force. The further from exactly in line compression bars are, the less end-to-end force you get from any given amount of handle squeeze.

Same with the zip line. If you actually do the truck experiment, you will find that the truck is easy to move until the zip line starts to get significantly far from straight. A 300 foot zip line with 6 feet of deflection is still close enough to straight to cause massive leverage effects.

So I think what's happening here is that you're not really taking into account the sheer scale of the forces you're playing with. Hanging 240 pounds off the centre of a 300 foot zip line will cause an enormous tension force inside the line, and the line will sag and the anchor points will give until the line deflects enough to get far enough from straight to reduce that force to tolerable proportions. And not all of that force will translate to cable stretch, either. We could easily be talking about enough force to crush tree bark, or haul the trunk sideways.

Ziplines done right have enough sag built into the design to reduce the line tension below the failure points of any of the components. If you're seeing component failure (permanent geometry change) after every few runs, your zipline's geometry is wrong.

Don't fight geometry. Geometry will win.
posted by flabdablet at 10:15 PM on June 2, 2012 [1 favorite]


On one end of the line I would install a large pulley with a 300+ kg weight one the other end. This can act as a self-tensioner

Something similar to this used on railway overhead lines
posted by jannw at 3:26 AM on June 3, 2012 [1 favorite]


I agree that turnbuckles are the optimal solution. I just read to expect a 2% sag in the cable which translates to 2 feet per hundred yards. Your line needs to be higher. Alternately you could always try this.
posted by Xurando at 6:00 AM on June 3, 2012


If you look closely at that railway line tensioner, you will notice that the cable being tensioned is would around a capstan with a much smaller diameter than the pulley the weight is hanging from. You will also notice that there's quite a lot of concrete in that weight. Now reflect that the purpose of that arrangement is to counteract the sag of a stretch of cable under its own weight only (as well as to compensate for thermal expansion and contraction). If your aim is to stop a 240 pound load from hitting bottom in the middle of a zipline run, you would need rather more weight than that or more mechanical advantage in the pulley and capstan.

That said, the hanging weight has the excellent advantage of applying a constant force regardless of position, precisely defining the tension in the cable regardless of cable stretch or mounting shift or zipline traveller load. If you load up the hanging weight enough to tension the cable to one fifth of its rated breaking strength, and your zipline crash test dummy doesn't bottom out half way, then you've got yourself a safe, self-tensioning zipline.
posted by flabdablet at 8:31 AM on June 4, 2012


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