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Prusik material safety???

J

JDUB007

New Member
Joined
Jun 27, 2019
Messages
27
As a newb in the saddle hunting world I’m wondering why everyone for safety is so adament on a 25 kN threshold for gear and using climbing rope but seems the prusik knot is the weakest link in the system. Are most of the companies and DIY saddle system builders using 8mm accessory cord? Or is this a stronger material?

I’m asking as I have some Sterling 8mm accessory cord from an older rope mod and wanting to know if it’s safe to use as a tether to bridge connection.
Thanks
 
Two 8mm lines are twice as strong as one. The prussic would have two lines for support.
 
bj139 said:
Two 8mm lines are twice as strong as one. The prussic would have two lines for support.
Click to expand...

This is NOT TRUE.

A Prusik loads Over and under the cord where it travels through itself on the wrap. They almost always break here, and it’s almost always at the MBS of the cord.

The knot in a tied Prusik only takes half the load, but it’s strength is doubled through the wrap on the biner. But This gets you back to MBS, NOT double.

Any Prusik will be rated for the MBS of the cord, or less. NEVER more, especially not DOUBLE.

Having said that, there are several friction hitch cords rated at 5000lbs+. If you’re not comfortable with the ~ 2500lb rating of typical accessory cord, there’s options. And they aren’t expensive.
 
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http://itrsonline.org/wordpress/wp-content/uploads/2015/02/004.Evans_.2014.pdf

‘Prusik breaking strengths were higher than the rated strength of a single strand of cordage by about 2 kN. However, the breaking strengths were not double the breaking strength of the cordage as has been suggested by some. Rather, the breaking strengths appear to be the strength of one strand of cord including the additional the manufacturer’s engineered safety factor. This means that for static system safety factor (SSSF) calculations, instead of using double the cordage strength, as is common practice, we must use the single strand cordage strength.’
 
bj139 said:
Two 8mm lines are twice as strong as one. The prussic would have two lines for support.
Click to expand...
I purposely worded my statement the way I did because I knew there would be strength loss due to knots and other factors. The 6mm cord I'm using for my prussic has a 7 kN breaking strength stated by REI.
 
So, Sterling Rope 6.8mm HollowBlock has a listed MBS = 3147, I have this ordered for my ATC auto stop/lock, sounds like this is also a viable option for replacing 8mm prusits that come on the TETHRD line belts, is that a correct assumption? What are the 5000 lbs. alternatives you were referring to above kyler1945 ?
 
Ok so I repeat, is it safe to use 8mm accessory as prusik material for a tether or linesman’s belt? Does it have adequate MBS?
 
I was hanging only supported on a prusik last week made from 6mm accessory cord from REI. One kN is about 220 lbs so the breaking strength of this is 7 X 220 = 1540 lbs. I have seen to divide by 1/3 for working load so 500 lbs. I think it is adequate for me. The tests Kyler posted showed breaking strength for a 6mm prussic was 9kN. Only you can decide for yourself.
 
JDUB007 said:
Ok so I repeat, is it safe to use 8mm accessory as prusik material for a tether or linesman’s belt? Does it have adequate MBS?
Click to expand...

A lineman’s belt is not considered life support - it will NOT prevent you from taking a quick trip to the base of the tree. It is a fall prevention device, not a fall arrest device. It is designed to create two points of contact on your hips so you can free your hands to work.

No one here is going to tell you what is adequate for you. There are general guidelines in the climbing world. These can be found with some research. The worst thing you could possibly do is just guess at what will work. The second worse thing you can do is trust some dude from the internet.

Talk to someone who is qualified to help you gear up, or take the responsibility into your own hands and do some research. There is plenty of information on this site that can be scrounged up with the search function. Lots of it is really poor advice, some is good. A quick google search will yield more reading material than you can possibly get through. Take some time to educate yourself my friend, and please stay safe.
 
8mm armor-prus has good strength rating. I have used it last season and I am a “big boned” guy. No issues with my setup.

https://www.treestuff.com/store/catalog.asp?item=1941

You really need to research, as you alone will experience the consequences of your decisions.

When I hear “accessory cord”, it does not give me confidence.
 
Here is my 6mm accessory cord on my 8mm tether. Notice I am adequately backed up.:)
I would be more concerned about a Ropeman 1 since testing showed it severed the rope at 4kN. In spite of this people still use them. I keep mine as a backup ascender. I have one of those Beal prusik cords (22kN )on my linesmans belt and am thinking I will reverse my tether prusik and my linesmans prusik.

 
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