View unanswered posts | View active topics It is currently Sun Jan 21, 2018 9:58 am



Reply to topic  [ 13 posts ] 
 Are your prusiks safe? Do you know? Report from On Rope 1 
Author Message
Rogue Engineer
User avatar

Joined: Mon May 14, 2007 4:26 pm
Posts: 2014
Location: Chattanooga
Post Are your prusiks safe? Do you know? Report from On Rope 1
Yesterday on the way home I realized I hadn't bought any climbing gear so far that day and to remedy the intolerable situation of having a little extra cash on me, I altered my normal course and went by On Rope 1. For some reason, I've become enamored with lightweight climbing rope, so I bought 150' of 9mm PMI EzBend - it has a MBS of 4720 lbs and only weighs 3.5 lbs/100 ft, but then that's another thread - coming to a forum near you soon.

Bruce was loading containers in prep for a big show, and he said he wanted to show me something. He went into the back and reappeared shortly with this short piece of 8mm PMI cordage that looked awful. The cover was burned and torn through to the core. The core was severely exposed but undamaged. I asked what happened - here's the story:

Bruce was on a rescue and they had set up a Z rig to raise a man in a litter; total weight about 300 lbs. The rope was rigged through a redirect via a prusik minding pulley (PMP), with a 3 wrap prusik (3WP) to capture progress. The Z rig was then attached to the rope via the 8mm prusik cord with a 3WP. When Bruce gave the signal to haul, three men pulled on the Z rig and the rescuee (and litter) didn't raise. What did happen was the prusik failed to grip and slipped down the rope under a lot of force, which explains the melted cover.

I was pretty much in disbelief that a 3WP wouldn't hold 300 lbs – in fact, I said to Bruce, probably without thinking, “I don’t believe it.”, emphatically I might add. Bruce explained what happened. This was a prusik that had been used before, and the rigger did not perform a prusik check before installing the prusik. Do you know how to check a prusik? I didn't, and I was quite alarmed that as much as I have used prusiks for life support, I wasn't aware of the condition Bruce was about to explain.

How to check a prusik:
You are really checking the cordage, not the prusik per se. You take the cord and form a bight. You squeeze the bight right at the bend between your thumb and index finger. A cord in good condition should all but close the eye with moderate pressure. An unsuitable cord will offer significant resistance to closing and will leave a relative large eye. If your cordage won’t pass this test, corrective action is simple – cut it up and dispose of it – it is dangerous. This stiffness prevents the cordage from collapsing tightly on the rope and hence does not hold securely.

Why does the cordage get so inflexible?
Dirt. Bruce says that the cordage gets accumulated dirt in it and gets stiff. Although, I‘ve seen milking cordage make it stiffer also. He went on to say that if the cordage gets drug through water and mud, its integrity and security degrades quickly.

Bruce is supposed to be sending me a full report, complete with pics. I asked if I could post the report and he said yes, so I’ll post them when I get them. Since he’s going out of town for a show, he may not get it to me until he gets back.

Another issues:
Another issue that came up was the security of a prusik on a tensioned rope. This is the exact case Bruce had in this rescue and is quite common. In essence, a prusik doesn’t grip nearly as well on a tensioned rope as one with a loose tail. In this very example, the rope was loaded with 300 lbs and a Z rig was attached to the tensioned rope with a 3WP. Bruce explained that prusiks work best when they can put a little bend in the rope – you’ve probably seen that if you’ve used prusiks much. The tensioned rope doesn’t allow that and hence the prusik doesn’t hold as securely.

Another issue surfaced when I asked why he didn’t attach the Z rig with a Rescuescender. He said that’s what they wound up doing in this case, but generally Bruce does not allow ‘steel’ on rescue lines. The reason is that a prusik will slip before damaging the rope where a steel device may seriously damage or sever the life support rope.

Another question was why not use two prusiks and Bruce’s answer was both revealing and intriguing. It seems a guy ran a bunch of tests on prusiks and found that a single 3WP would start to slip at about 1575 lbs. Adding a second prusik only increases the holding power to about 1800 lbs or so. So very little is gained by a second prusik. Then a load sharing prusik was tested. This is where the two prusiks are on the same cord and are connected via a biner between the two prusiks. The same testing revealed an increase in holding power to about 3150 lbs – just about double a single prusik.

Interesting stuff, huh?


Wed Oct 06, 2010 7:00 am
Profile

Joined: Wed Apr 25, 2007 10:11 am
Posts: 855
Location: Dawsonville, USA (north of Atlanta)
Post 
That is extremely interesting stuff. I'm going to do that test today on any cordage loops that I have used in a life-support situation.

Although it doesn't affect the footloop that a lot of new climbers use (since it's only a climbing aid), but it's obvious from Bruce's tests that the 3-wrap Prusik begins to slip at a much lower weight than I had suspected when used in a life-support system.

Here's what I've been doing for about six or seven years when climbing SRT with a Texas rig (two ascenders). I often tie a long safety loop above the upper ascender as a backup. Instead of a 3-wrap Prusik hitch I have been using a Klemheist hitch because it slides up easier when not weighted but seems to grip faster when a load is applied.

My question for Ron or Bruce: Since the Klemheist stretches out a tiny bit when weighted, would it likely hold a higher loadwork before slipping than the 3-wrap Prusik?


Wed Oct 06, 2010 7:36 am
Profile WWW
Rogue Engineer
User avatar

Joined: Mon May 14, 2007 4:26 pm
Posts: 2014
Location: Chattanooga
Post 
WildBill wrote:
That is extremely interesting stuff. I'm going to do that test today on any cordage loops that I have used in a life-support situation.

Me too!

WildBill wrote:
...Although it doesn't affect the footloop that a lot of new climbers use (since it's only a climbing aid), but it's obvious from Bruce's tests that the 3-wrap Prusik begins to slip at a much lower weight than I had suspected when used in a life-support system.

Hmmm, well, a 3WP holds about the same load as toothed ascenders, slightly more IIRC, the difference being in what happens when they do start to fail - one dreadfully shreds the rope and the other slips.

WildBill wrote:
...Here's what I've been doing for about six or seven years when climbing SRT with a Texas rig (two ascenders). I often tie a long safety loop above the upper ascender as a backup. Instead of a 3-wrap Prusik hitch I have been using a Klemheist hitch because it slides up easier when not weighted but seems to grip faster when a load is applied.

Hmmm, yeah, prusiks can tighten up.

WildBill wrote:
...My question for Ron or Bruce: Since the Klemheist stretches out a tiny bit when weighted, would it likely hold a higher loadwork before slipping than the 3-wrap Prusik?
That's an excellent question; I don't know the answer, but somewhere I've seen 'holding' tests on friction knots, done by Tom Mosier as I recall, but I don’t recall the results.

Bruce would likely know, but he pretty much uses 2WPs for personal and 3WPs for rescues and highlines.


Wed Oct 06, 2010 8:59 am
Profile

Joined: Fri Feb 13, 2009 8:25 pm
Posts: 98
Location: Trenton, GA
Post 
Thanks for the good info Ron. Check out these Youtube videos by NZcaver. He has a few testing prusiks and rescucender.

http://www.youtube.com/user/NZcaver#g/u


Wed Oct 06, 2010 10:20 pm
Profile
Rogue Engineer
User avatar

Joined: Mon May 14, 2007 4:26 pm
Posts: 2014
Location: Chattanooga
Post 
Thanks Mike, some interesting vids without a doubt. Some are quite short; I wish they were a little longer and included a little more detail though.


Thu Oct 07, 2010 9:44 am
Profile

Joined: Wed Dec 12, 2012 1:56 pm
Posts: 1
Post Re: Are your prusiks safe? Do you know? Report from On Rope
Hi Ron:

I came across this post through a search and I read this part of your post:

Ron wrote:
Then a load sharing prusik was tested. This is where the two prusiks are on the same cord and are connected via a biner between the two prusiks. The same testing revealed an increase in holding power to about 3150 lbs – just about double a single prusik.


I am interested to know what this 'load sharing prusik' set up looks like. Are you referring to a tandem prusik configuration as shown in this picture: http://cdn1.bigcommerce.com/server600/a ... 00.600.jpg

If not then would you mind sending me a link to a picture of this type of set up? would you also happen to have any information on where those tests where done?

Thanks for your help on this!


Wed Dec 12, 2012 5:21 pm
Profile

Joined: Thu Jul 09, 2009 4:23 pm
Posts: 86
Location: Buffalo, NY
Post Re: Are your prusiks safe? Do you know? Report from On Rope
Ron - Once again, many, many thanks for these contributions. While many of us get lost in the clouds of gear lust and dreaming of ever-higher trees, you have a much-appreciated knack of keeping us grounded in what really matters, the safety consciousness that allows us to return to the ground so as to climb another day. I can't thank you enough.

_________________
Be the person your dog thinks you are.


Wed Dec 12, 2012 7:16 pm
Profile
User avatar

Joined: Wed Apr 25, 2007 10:25 am
Posts: 4375
Location: Jamaica Plain, Massachusetts
Post Re: Are your prusiks safe? Do you know? Report from On Rope
Since we've been talking on the forum a lot recently about SRT hitch climbing the prusik cord safety question is again becoming very relevant. I don't know what kind of cordage Bruce uses for his rescue prusiks so I can only talk about what type of cord folks are using for SRT hitch tree climbing and what a good pre-climb hitch inspection protocol is.

Cordage material and construction for SRT hitch climbing
Favored hitch cord types are kernmantle construction with heat resistant covers (jackets) and high density/strength polyester type core strands. Examples are: Beeline 8 and 10mm (different material for the two different sizes), Armour-Prus 8 or 10mm, Ocean Polyester 8 or 10mm and others like NE Ropes HRC. Tenex is an example of a cord material that performs very well as a prusik but has a very low melting temperature.

So with these cords you need to know your cordage materials and know what to look for on pre-climb inspection. For example Tenex is a single braid 12-strand cord (not a kernmantle) and has to be inspected very closely for signs of melting. With no cover the load supporting part of the cord is acutely vulnerable to failure from melting. Climbers who favor Tenex understand its characteristics and inspect and climb on it accordingly.

Pre-climb inspection and testing
Cordage used for tree climbing hitches does not typically get the same type of dirt abuse you might see in caving or non-tree climbing rescue scenarios. Probably the number one cause of hitch cord deterioration is overheating and wearing of the cover fibers through repeated use or abuse, such as descending too quickly on the hitch. All tree climbing hitch cords eventually get some amount of glazing through normal use and the eventual breakdown of fibers in the cover. Before the fibers start to seriously break down glazing becomes an issue first. A heavily glazed hitch cord will not grip the rope very well. Heavy glazing is detectable by visual inspection and by kneading the cord between your fingers. If the cord does not feel supple and soft when you roll it between your fingers it is potentially no longer suitable for use. The touch inspection should also work through the length of the cord to verify consistent diameter and feel of the cover and core. There should be no core gaps felt through the cover. Visual inspection should also look for excessive broken strands in the cover and core herniating through the cover. Slight glazing (for kernamantle hitch cords with heat resistant fibers) is normal and usually starts appearing on new hitch cords within a couple of climbs depending on how aggressively the climber uses the hitch.

After visual and touch cord inspection the number 1 safety step for SRT hitches is to tie the hitch on to the climbing rope and attach it to your harness. Slack the hitch on the rope, this means fully collapsing/compressing the hitch downward so there is no tension on the climbing rope. Grab the climbing rope above the hitch and pull it sharply upward. The hitch should grab quickly and securely. If it does not it is not safe to climb on. There are three possible reasons that this unsafe condition can occur: 1. Configuration problem (the hitch is not tied correctly or the cord type or length is not well matched to the climbing rope) 2. Excessive glazing or friction wear (the cord is no longer capable of gripping the rope) 3. The cord has stiffened due to UV exposure, glazing or aging and will no longer bend to a tight enough radius to effectively grip the rope. This is essentially the same problem that Ron referred to in his description of the prusik cord failure to grip in the rescue. The cord is not safe to climb on if it cannot form a tight enough radius to grip the rope.

Assuming you are using a cordage type that it well accepted for tree climbing and you perform a good visual and touch inspection, and the slacked hitch test, you will have a solid basis to expect safe hitch performance for your SRT climb.

Did I miss anything?
-AJ


Tue Dec 18, 2012 5:54 pm
Profile WWW
Rogue Philosopher
User avatar

Joined: Wed Dec 09, 2009 8:42 pm
Posts: 250
Location: OhighO
Post Re: Are your prusiks safe? Do you know? Report from On Rope
fascinating, and critical info shared by both Ron, and moss. As tree climbers hanging from ropes, our psychological security often depends on only two things; our climbing rope, and TIP. We split tail climbers definitely need this focus on our cord to sharpen, it is after all, our connection to those two most obvious factors.

Quote:
Why does the cordage get so inflexible?
Dirt. Bruce says that the cordage gets accumulated dirt in it and gets stiff. Although, I‘ve seen milking cordage make it stiffer also. He went on to say that if the cordage gets drug through water and mud, its integrity and security degrades quickly.

Bruce is supposed to be sending me a full report, complete with pics. I asked if I could post the report and he said yes, so I’ll post them when I get them. Since he’s going out of town for a show, he may not get it to me until he gets back.


This leads me to ask, why not wash your prussik cord?
I have never washed my split tails, but I am thinking of giving it a go, and comparing performance afterward.


Quote:
Did I miss anything?


Thanks moss, for putting this in a tree climbing perspective, regarding split tails, especially in SRT, where the cordage can be your only connection to the climbing rope.
The only thing I would add is for those of us climbing on spliced eyes -Inspect the eyes! The eyes are the most vulnerable part of the spliced, e2e split tail.


Wed Dec 19, 2012 10:05 am
Profile
User avatar

Joined: Wed Apr 25, 2007 10:25 am
Posts: 4375
Location: Jamaica Plain, Massachusetts
Post Re: Are your prusiks safe? Do you know? Report from On Rope
Yep, thanks for the backup Hook, inspect your tied, sewn, or spliced cord eyes!
-AJ


Wed Dec 19, 2012 12:29 pm
Profile WWW
User avatar

Joined: Wed Apr 25, 2007 10:25 am
Posts: 4375
Location: Jamaica Plain, Massachusetts
Post Re: Are your prusiks safe? Do you know? Report from On Rope
hook wrote:
This leads me to ask, why not wash your prussik cord?
I have never washed my split tails, but I am thinking of giving it a go, and comparing performance afterward.


My hitch cords never seem to get dragged through the mud ;-) Typical scenario is pine pitch which can stop the hitch from functioning, ie: it grabs too tight and won't release if the glob is big enough. There have been dozens of solution suggested to solve this: mayonnaise, peanut butter, olive oil etc. I give the hitch a good wiping down and keep using it, the pitch melts away from the friction heat of normal climbing. So... I haven't yet found a good reason to wash my hitch cords. They seem to wear out from normal use before any kind of dirt build-up occurs.
-AJ


Mon Apr 08, 2013 11:49 am
Profile WWW
Rogue Canuck
User avatar

Joined: Wed Apr 25, 2007 7:56 pm
Posts: 897
Location: British Columbia, Canada
Post Re: Are your prusiks safe? Do you know? Report from On Rope
I like to keep my cordage clean -- I have ever since diagnosing a slipping Blake's hitch as dirt-related, way back when I first started. Ropes and cords always come out softer and fuzzier and, I think, better functioning. The Springtime-fresh smell is a huge bonus, too! :wink:

It's true that our cordage doesn't get dragged through the dirt like caving stuff, but it still gets pretty grubby, what with sweat and resin and the constant shower of powdered bark, moss, lichen and leaf mulch. Just think, all that stuff you rinse down the shower drain after a climb? It settled all over your gear, too!


Mon Apr 08, 2013 4:51 pm
Profile
User avatar

Joined: Wed Apr 25, 2007 10:25 am
Posts: 4375
Location: Jamaica Plain, Massachusetts
Post Re: Are your prusiks safe? Do you know? Report from On Rope
Dietley wrote:
The Springtime-fresh smell is a huge bonus, too! :wink:


True!


Mon Apr 08, 2013 7:02 pm
Profile WWW
Display posts from previous:  Sort by  
Reply to topic   [ 13 posts ] 

Who is online

Users browsing this forum: Yahoo [Bot] and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
cron
Powered by phpBB © 2000, 2002, 2005, 2007 phpBB Group.
Designed by STSoftware for PTF.