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Use of Prusiks vs. Klemheists in Highline Tension and Elsewhere
Any thoughts on the use of a klemheist knot instead of a standard prusik for tensioning or in minding pulleys at height? The klemheist is much quicker to tie on a line and stays neater, but outside of climbing I've never seen it used on static rescue lines. I'd like input as to it's reliability in this use? Ideas, Thoughts, Comments?
Thanks,Tom
Thanks,Tom
Replies to This Discussion
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Permalink Reply by Ben Waller on -
The reason for using prussics is that they slip if they get over-tensioned. Klemheists have a firmer grip on the rope, and are more prone to over-tensioning. This is a particular problem with highlines.
The other issue is that if you're using a a double track line in your highline system, it is much easier to pull even tension with prussics than with klemheists.
When you say "minding pulleys" at height, do you mean using prussics with a prussic-minding pulley in a belay system?
If so, the reason for using prussics is that they are easier to hand-tend if belaying a lowering system.
The self-belay feature only works when raising loads, but most vertical belays are used to back up lowering a load.
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Permalink Reply by Thomas J. Harty on -
Ben that makes sense. Do you mean that a klemheist would not only lead to more tension being possible but would be more difficult to release as well. On a highly tensioned line a prussic on a 3:1 can be pretty tough to loosen, I would think a klemheist would release easier but I have not tried it as I had not resolved the issue of their effectiveness yet, hence the post! Thanks, i'll stick with prussics.
Tom
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Permalink Reply by Ben Waller on
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Thomas,
Our state system uses 3:1 prussics for all vertical belays, highline, and swiftwater rescue tension diagonal applications. They've worked well, and we've never had a system failure when using them.
I'm following the "If it's not broken, don't fix it" plan on this one.
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Permalink Reply by Paul Montpetit on
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you got it Ben....well done
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Permalink Reply by JamesNC on -
I will also add that two triple wrap prussics will start to slip between 3000 and 3500 lbs so you know also the amount of force being applied to your system. I'm sure you could do enough test on the klemheists to come up with the lbs also but the figures are already there for the prussics.
James
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Permalink Reply by Thomas J. Harty on
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James, that is a great piece of data, thanks! We are researching methods for some failure testing experiments with the students. It is not as simple as it might seem to get the measurements, but I bet if we can do it that it will prove to be a spectacular and memorable thing to witness a planned failure (from a distance) under load! I have a feeling the students will then understand why we drill them so much on this!
Thanks again for the data point.
Tom Harty
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Permalink Reply by Ben Waller on
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James,
That's debatable. Are we talking wet rope or dry? Muddy? Icy? New? Worn?
What diameter prussics? How long are the prussics?
What diameter of main line? How rough or smooth is the main line sheath?
Etc.
There are so many variables in this equation that I'd have to see some pretty strong evidence to back up those numbers before I'd trust them.
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Permalink Reply by JamesNC on
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Well if your looking for always and never Emergency services isn't for you but to anwser you questions that is from several studies for 8mm on 1/2 general use nfpa rope dry. New and rope in service that had been used several times in everything from highlines to general work. The numbers are a average. With all knots working off friction anything that makes the rope slippery will effect all knots and friction devices. The prussics were set up to be to be 2 inches apart the length of the sling it self was variable.
These test were done do to the fact we had seen numbers like this in several books and wanted to see how close they were in real life and all I can say is they fall right on.
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Permalink Reply by Ben Waller on
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"Well if your looking for always and never Emergency services isn't for you..."
What the hell have I been thinking for the last 35 years that I've spent as a firefighter and paramedic???
The parameters you provided exist in a very narrow window, and many real rope rescues don't occur under those conditions.
As for the numbers being an average, they are apparently skewed to something very close to ideal conditions - which you can't count on during a real rescue. That can lead to a false sense of security.
Did you calculate the kN forces, or did you have a way to measure those forces?
So, did you measure the forces with a load cell? What kind? How recently had it been calibrated? Was the failure do to overtensioning, a shock load, or some other kind of force?
What I'm trying to determine are the test conditions.
And James, I didn't say "always" or "never". You did. I'd appreciate it if you'd stop putting words in my mouth and stick to the questions I asked.
Thank you.
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Permalink Reply by JamesNC on
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I'm sorry i didn't mean to offend you and I didn't quote you but no test can be done under every condition we encounter in the field. I made that statement because so many people are looking for black and white today. My Bad
The forces were measured with a dynamometer during a slow pull as you would have on high lines during rigging not shock loads. I thought that info was relative to the question of tension use of prussic
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Permalink Reply by Ben Waller on
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It was, but there are other tension uses of prussics - hauling loads vertically, belay stops of rappels or lowered loads, capturing progress on boat telfers in swiftwater applications...etc.
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Permalink Reply by George B. Gardner III on -
Tom,
As both a firefighter/rope rescue tech and a recreational climber, my concern would fall with the increased grip a klemheist has on the main line. Judging for my experience with both knots, I would be inclined to say that should the system be shock loaded, prusiks would have more "give" resulting in a reduced force applied to the system, where the klemheist would likely hold tighter increasing your chance of a catastrophic-system-failure.
As an example, I took a lead fall rock climbing a few years ago on a prusik belay system, I was a little rattled and slightly bruised but otherwise fine. A few weeks later, a friend (almost same weight and build) took an almost identical lead fall. His preference had been, until that day, a klemheist belay. His belay system failed in the 8mm cord of the klemheist and he fell nearly forty feet before being stopped, by other climbers grabbing his rope, ten feet from the ground (inverted, no less!). He has since switched to prusiks.
This is solely based on personal experience with both knots, and personal preference. I would be quite interested in any data regarding this subject you collect when you test them both out.
Stay Safe (and only slightly sane)!
-George
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