Prusik talk; slipping, melting, breaking?


Original Post
Firestone · · California · Joined Nov 2015 · Points: 449

Hi all, I'm turning to MP because I need a better idea about the material properties of some climbing gear. This post is actually slackline related but I hope it can evolve into a discussion about the material properties of different soft goods.

I will be using a pair of prusiks to tension a piece of flat nylon webbing. I want to use the right kind of material for the prusik, I am leaning toward two thin dyneema slings or loops of Kevlar titan cord.

What would the best material be for the highest coefficient of friction with flat nylon webbing rolled around 9.8mm climbing rope? The webbing is Type-18 nylon.

Jim Titt · · Germany · Joined Nov 2009 · Points: 0

I doubt there is any information on the coefficient of friction of Kevlar on Nylon but as it (and Dyneema) are slipperier against themselves there´s no reason to suspect they would be any better than nylon to nylon which has a suprisingly high coefficient.

The rescue guys do a lot of high-load testing on Prusiks so they would be the area to search.

Firestone · · California · Joined Nov 2015 · Points: 449

I'm just worried about the slip of the line so maybe I should go with 7mm nylon prusiks wrapped around the webbing and a thick boat rope.

What got me thinking was recently I was using a kong duck ascender to tension a long slack line. I couldnt get it to full tension with the duck because the cam wouldn't bite once the static rope I was using to tension the webbing became too taught. It slipped along the rope without biting so I switched to a prusik and was able to finish tensioning.

Nick Hatch · · Unknown Hometown · Joined Jun 2017 · Points: 0

If you're trying to reduce slip, the most important thing will probably be the relative size of the friction cord compared to the rope. (eg 8mm hitch cord on 11mm rope is generally accepted to work well.) 7mm cord is dirt cheap, I'd give that a shot before stepping up the size of the rope or trying more exotic materials. 

Arborists deal with this stuff a lot too - lots of hitch cord materials in use with heat-resistant and high-friction sheaths. I would be shocked if you could get 8mm Ocean Poly on a thicker rope ( Like Samson Vortex / Voyager ) to slip. Sterling RIT is a cord which is very squishy and compliant, may help grab the webbing-rolled-rope. Lots of choices. 

If you want to do more reading:

Ropes and Friction Hitches used in Tree Climbing Operations

Protecting An Abseil: A Study Of Friction Knots

Ch 9 PRUSIK KNOTS - Lyon Report

20 kN · · Hawaii · Joined Feb 2009 · Points: 1,128
Firestone wrote:

Hi all, I'm turning to MP because I need a better idea about the material properties of some climbing gear. This post is actually slackline related but I hope it can evolve into a discussion about the material properties of different soft goods.

I will be using a pair of prusiks to tension a piece of flat nylon webbing. I want to use the right kind of material for the prusik, I am leaning toward two thin dyneema slings or loops of Kevlar titan cord.

What would the best material be for the highest coefficient of friction with flat nylon webbing rolled around 9.8mm climbing rope? The webbing is Type-18 nylon.

If you're trying to build a LineGrip for slacklining just buy an actual LineGrip. The forces in longlining are too high to be making stuff up, you're going to lose a finger or destroy your piece of T-18. Anyway, the thinner and more flexible the cord is relative to the base material, the less likely it is to slip. 9.8mm rope on 1" flat webbing is not going to be very secure. Dyneema would be a very poor choice as it's extremely slipperly. Kevlar Titancoard is stiff but might work.

Old lady H · · Boise, Idaho · Joined Aug 2015 · Points: 120
20 kN wrote:

If you're trying to build a LineGrip for slacklining just buy an actual LineGrip. The forces in longlining are too high to be making stuff up, you're going to lose a finger or destroy your piece of T-18. Anyway, the thinner and more flexible the cord is relative to the base material, the less likely it is to slip. 9.8mm rope on 1" flat webbing is not going to be very secure. Dyneema would be a very poor choice as it's extremely slipperly. Kevlar Titancoard is stiff but might work.

Agreed. Stick with products meant for slacking.

But, Google prusik belay backup. That should get you to rescue rigging that fire departments, etc. use. What they use the prussik for is to back up the rope lowering a litter. A "belayer" is assigned to tend the prussik, and if the lower somehow gets away, they let go so it can grab. I assume it is anchored separately from the lowering rope. It would be subject to a rope sliding through, and would need to resist that, yet grab. Similar to what a prussik rappel back up does, but anchored differently, and tested differently also.

Best, H.

eli poss · · Durango, Co · Joined May 2014 · Points: 136

Dyneema is probably the worst option as it is quite slippery and also has a very low melting point. Aramid cords tend to have very high friction, but come with their own set of problems such as the fibers actually abrading against themselves.

From my experience using it for various applications on a zip line, Technora cord (IIRC a blend of aramid and dyneema) provides very good friction and performs well in use with a 3 wrap prusik hitch. It is also very resistant to abrasion and has extreme, mind boggling resistance to cutting (we use it to control speed on a zip line and it moves along a steel cable at high speed many times without any sign of cutting any strands).

The only issue is foresee, however, is that its strength loss when used in knots or hitches is alarmingly high. It's strength efficiency when tied in a figure 8 is 40%. 7mm accessory cord was 92% efficient in the same test. Without fully understanding your application with slacklines, I can't make a suggestion for material, but based on your desired material properties, technora and nylon fit the bill.

Gavin Towey · · Unknown Hometown · Joined Oct 2015 · Points: 0

With the forces involved in slackline setups I would be worried that a prusik would simply cut the nylon webbing.

Firestone · · California · Joined Nov 2015 · Points: 449

Awesome, great info everybody. Apparently before the line grip in slacklining people used prusiks. It should be noted the prusik is used to tension the line and once the line is tensioned all the weight will be transferred directly onto the anchor.

I think nylon sounds better, like materials will probably behave more suitably. If anyone is interested I may post my results from tensioning.

grog m aka Greg McKee · · Unknown Hometown · Joined Aug 2012 · Points: 0
If anyone is interested I may post my results from tensioning.

Yes please. 

Jonathan Croom · · Unknown Hometown · Joined May 2015 · Points: 95

Hey Firestone, I have actually used a similar setup before I bought a Linegrip, so I'll add what I've learned. 

First, the prusik system is workable for softpointing at low tensions. The trend (especially for highlining) is super low base tension, so if you plan to use a prusik for a Buckingham system through a weblock (3:1 MA ignoring friction) and tension just what one person could pull with that system, I think you're ok. One time I had a friend help me tension with that system with a prusik, and the edges of the webbing (also T18) melted the nylon prusik cord. The webbing didn't look great after that, but not so bad that I had to retire it. 

I would not use the prusik "linegrip" with a pulley system for softpointing. It simply won't hold at higher tensions.

I used three 6" long pieces of really old, fuzzed out dynamic rope as the core, and wrapped the webbing around them. I think I used both 6 an 7 mm nylon cord. More important than the size of the cord was using a LOT of wraps for the prusik, and taking the time to make sure the hitch is well dressed and neatly tensioned.

Interesting that you mention the problem with the Duck; I recently noticed the same problem (ascending a rope) and it was very annoying. 

Try tensioning with a soft release rather than dealing with the prusik. You probably already have to embed a soft release if you're using the Buckingham system so it's no extra gear, and tensioning with the soft release will work way better than the prusik. Also tensioning with a soft release allows you to use a double wrap in both weblocks, which is not a bad idea to reduce  (not eliminate) the effects of slippage at low base tensions.  

If you are looking for a less expensive linegrip, look at this: http://ambaradam.net/product/aurrera/. I built a linegrip of this style, and it worked fine at lower tensions, but slipped at higher tensions.

When you decide you need a real Linegrip, order from Europe. I got mine from Slacktivity, and it was cheaper than US even with shipping.

Jonathan

Ian Hill · · Unknown Hometown · Joined Jun 2017 · Points: 0

A buddy and I used a bow fishing bow to sling a slack line over a gorge. As neither one of us can slackline very well, our goal was to set hammocks from the line. We cut an old bike inner tube up and placed it between the sling and the webbing we used as the slackline. This helped with prusik slip and kept the hammocks reasonably taut. Nylon is slippery stuff.

Guideline #1: Don't be a jerk.

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