UKC

Hi

This may be an odd post, but i figured this was the best place to ask....

I am looking for a rope, it is for a Cable Camera system, and through google etc it seems the best kind of rope to use is Static rope.

I did have some rope with Dyneema? core given, which i believe was some sort of sailing rope, but when it is tied between 2 points, it just continually stretches, i had about 100m between 2 trees, head height, tightened up with ratchet strap, and when the camera was on it ( aprox 4kg ) it was hitting the floor in the middle

Would static rope eliminate this? And if so, what is a good kind to buy? aprox 100-150m length of it, and around 8-10mm thick

Thanks for the help, sorry its not really a climbing question!

In reply to Wayne Goldring:

http://www.bananafingers.co.uk/beal-antipodes-105mm-static-rope-metre-p-821...

http://www.rockrun.com/rock-climbing/climbing-ropes/beal-semi-static-rope-1...

that's the semi-stat rope most climbers use, it's alot less stretchy than dynamic rope but in your application 100m of rope will sag no matter what unlesss you put some serious tension on it I imagine.

Maybe there is a camera rigging forum out there somewhere that would be able to give better advice?

In reply to Wayne Goldring:

That's asking an awful lot of a rope IMHO. A 1.8m deflection over 100m isn't a lot for something that is inherently flexible.

In reply to Wayne Goldring:

Thank you. sadly very little about this on forums, anything ive found has been more people asking the same thing!

There is a figure on the ropes that relates to the stretch? so on one of them it had a 2% rating, is that as simple as thinking that 100m of it would stretch a further 2m under pressure? so i could expect that a 100m run would need to be 3m off the ground either end in order to account for the stretch?

In reply to Wayne Goldring:

You'll find that everything bends in the middle when you rig it like that (even scaffolding), it's a law of physics. I'm not sure exactly what type of rope you do want...

The more you tension the rope, the less sag you'll get, but just be aware that you can build up some astronomical forces on your trees when you do this.

In reply to Wayne Goldring:
You'll get a little bit of sag regardless, because zero sag means infinite tension in the rope. If you were able to buy a magically weightless and stretchless infititely strong rope, you'd just move the trees (or break your rigging) instead.

With a 4kg camera and 100m span, and with the rope sagging very little, the tension in the rope is more or less 200kg/(how far it's sagging).
So, 1m sag -> about 200kg worth of tension, 10cm sag -> about 2 tonnes.
(That's assuming you have that magic weightless rope, which obviously you won't have - so in the real world it'll be more severe even than that.)

The bottom line is that regardless of the rope, it's probably not going to be practical to rig the rope over such a long span at head height and eliminate the sag in the middle.

If you can raise the rope up a bit to deal with say about 1.5 - 2m sag you might well be able to use an abseiling rope. They are manufactured to be slightly stretchy though, to give them shock-absorbing qualities they need to cope (and help the rigging to cope) with minor falls, jerks and shock loads that are common the way they're used in climbing and caving.

There is one rope on the market (which isn't all that easy to find) called "Black Marlow" - it's a lot like a climber/caver's semi-static, but is less stretchy. You won't find it in a climbing shop because it isn't CE approved for civilian climbing/caving use. (It's insufficiently stretchy to meet the EN1891 standard.)
It's made for the military to use for very long abseils and other gun-toting "hut hut hut hut" type activities. Still stretches a little bit though.
http://marlowropes.com/defence-products-1/abseilclimbing/abseil-rope.html

Commercial camera rigs ('Skycam' type things) usually use something like Vectran. Eg: http://marlowropes.com/film-a-theatre-prod/v12.html
Rather expensive.

Otherwise, you could perhaps look at using a steel wire rope. Say about 6mm ish, maybe 4mm. Awkward to handle, surprisingly easy to kink, ideally needs to be terminated with clamps ('dogs'/wedge sockets/Strandvise/Reutlinger, etc... ) rather than simply tying a knot, and if you're using climbing pulleys it'll probably eat your sheaves. But it is un-stretchy and relatively cheap.
(And it will still sag some in the middle of a 100m span.)

In reply to Wayne Goldring:

No. If I'm remembering right the test begins with a 50kg load on the rope, and then that 2% is how far it stretches when a further 100kg is added.

The rope is kinda sorta elastic, so Hooke's Law applies (ish). (Hooke's law is the one that says the elongation of an ideal spring is proportional to the load on it.)

So (again - if I am remembering right), you can expect your 100m rope to stretch about 2m with 100kg on it, but more like 4m with 200kg on it.


No, the stretch is in the length of the rope. So if you have a perfectly flat weightless (ignoring for a second that isn't actually possible) horizontal 100m rope, and the rope stretches to 102m long with a load in the middle, it'll sag by [Square Root of (51 squared - 50 squared)], which is about 10m!

In reply to Wayne Goldring:

Bottom line is you're asking a lot.

Climbing cord isn't cheap http://www.needlesports.com/Catalogue/Rock-Climbing-Equipment/Cord-Tape/Cor... Ideally of the stuff they stock I'd go for the 5.5mm Dyneema to keep the rope weight down but it's pricey.

Steel wire, in the 4mm range is probably your best bet but to really reduce the sag you're also going to have to limit the trees bending and be really careful not to overload the wire, if 100m of it fails under tension it could easily kill someone.

The best solution is lose some camera weight. Failing that film multiple takes over shorter runs or add intermediate supports.

jk

In reply to deepsoup:

Oops. Too late to edit, but before anyone else jumps in to point it out - stupid mistake there:


Actually that should be 100kg / vertical distance it's sagging
So 1m sag -> about 100kg tension, 10cm sag -> about 1tonne

(If anyone wants to check my working, I'm assuming a weightless rope and taking moments around the anchor, then assuming that the rope is near enough horizontal that the horizontal component of the load on the anchor is near as dammit the same as the tension in the rope.)


Still true. Actually, the weight of the rope will probably make my original figure nearer the right answer than the corrected one. 100m of semi-static will probably weigh about 6-8kg, and the distributed self-weight of the rope will contribute a roughly comparable load to the weight of the camera in the middle.


Hm. I think that is still about right (ish).