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Zipline Cable Redundancy

7x19 Aircraft Cable Cross Section

I've had quite a few of you ask me why a zipline cable, when there is only one of them (typically!), is structurally redundant when a component like a turnbuckle isn't. That's a great question - and there is a good answer to it.

The type of cable used in most zipline installations is called aircraft cable (which, despite the name, aren't used in aircrafts). There are several types of aircraft cables, but the specific cable used for several reasons is usually 7x19 aircraft cable. The photo attached to this article shows what this cable is made up of - 7 bundles of 19 smaller cables twisted into one large cable. Take a closer look next time you ride - you can see the smaller cables making up the one zipline cable.

Do you see the redundancy in the cross section? Remember that the issue with the turnbuckle was that there is no structural redundancy - if any of the three components that make up the unit had a hidden flaw in them, the entire assembly (and the zipline you are one) would break.

The cable might look similar - there is only 1 cable after all. But the difference is that the LARGE cable won't have a manufacturing flaw - only one of the smaller cables in one of the 7 bundles of 19 smaller wires would! If one of the wire strands has a flaw in it and it breaks, there is still plenty of redundancy in the other 132 wires that combine to make up the cable you are zipping down. So, the one zip line cable has built in structural redundancy - that's why professionals use it. As a side note - this is very similar to how cables are constructed for redundancy on cable-stayed and suspension bridges!

The Nature of Adventure

I just watched Nik Wallenda of the Flying Wallenda fame walk across the mouth of Horseshoe Falls. It was an incredible showing of focus of mind, athleticism, determination, and stick-to-it-ness (he actually had to get officials both in the US and Canada to change law to pull the stunt off!) that, if you missed, is at least worth a few minutes of highlight reels.

I will say, though, that one of the things running through my mind as I watched this event was a line that always spoke to me from Mr. William's The Glass Menagerie. It's a line of Tom's from Scene 6. Badly taken out of context, here it is:

Hollywood characters are supposed to have all the adventures for everybody in America, while everybody in America sits in a dark room and watches them have them !

I'm not running out on a wire rope across a 200 foot waterfall anytime in my lifetime (and I suspect the same is true of you!). But I hope the opposite is true- that the best adventures of your life aren't spent watching other people (or fictional characters!) have all the fun. It's a small part of why I've decided to get out of my comfort zone in engineering, start up a business, and bring adventures to other people. If, in whatever small way, I can help people to understand, push against, and start the process of learning to overcome their own limitations, well, that's a meaningful thing... and a worthwhile adventure.

Turnbuckles and you

The typical turnbuckle

Wow - an entry about turnbuckles. I can hear the Internet right now paving its way to this page...

Page views aside, it's important that I say a few things about this otherwise humble piece of rigging hardware, if only for one simple reason.

Each and every home base zipline kit I have seen to date includes a turnbuckle on the main cable to allow for minor adjustments to the cable tension.

Turnbuckles are certainly convenient – they do what they are designed to do in that they allow for small changes in the tension applied to a wire rope (the zipline itself). This is handy for picking up the line a little bit to get a bit more speed out of your line, or for letting out the line down a little due to some tree growth when you prep your line at the start of a new season.

But take a look at the picture of the turnbuckle attached to this entry. The basic vanilla turnbuckle consists of three parts, two bolts on either end and a collar or frame that ties them together. In a zipline installation, both of those bolts are going to be attached to the wire rope of the zipline, making the turnbuckle a critical part of the system. The turnbuckle in this case becomes a mechanical joint tying two pieces of wire rope together - the same wire rope that you are thinking about riding down.

However, it is the three separate parts of this component that lie at the heart as to why you don't want to see a turnbuckle on a zipline that you're thinking about riding. The reason for this is that any structural system to which you are trusting your safety to (and a zipline is in that category!) is one you want to see redundancy on. A redundant structural system is one in which if a member fails, there is a failover system that will engage to keep you safe. In the case of the turnbuckle, not only is there no redundancy in the turnbuckle itself, there are instead actually 3 points of total failure. In other words, if any of the 3 parts (either bolt or the collar) were to fail, the tension in the wire rope goes to 0 and the wire (and anyone on it) is going down.

"But, Dan, certainly these turnbuckles are rated for more than the weight of a zipliner and cable! What are the odds of a turnbuckle failure?" I would suspect that the first part is true – the turnbuckle probably has a working load limit that is more than the weight of the rider and cable (obviously – I didn't check your system!). But here's a thought – how sure are you that the turnbuckle itself is good? How do you know that there isn't a small fracture internal to the steel bolt or the collar that you can't see? Remember – if ANY of the 3 pieces give way, the ENTIRE turnbuckle fails, resulting in a failure of the ENTIRE system.

Caveat Emptor- Buyer Beware.

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