Progress Capture Testing for a jigger

by | Jul 16, 2018 | Rope Rescue |

Progress capture testing for a jigger

I was about to post an article about the jigger (aka a set of fours) – how to build (and troubleshoot) using a couple of small double pulleys and cord (coming soon). I tried to remember if I had ever tested the progress capture Prusik as this would be good to go into the post. There was a memory of testing over ten years ago and but couldn’t find anything recently. What I am talking about is testing a 6mm tied with a Prusik hitch on an 8mm cord.

I thought it was just going to be confirmed the 3 on 2 Prusik was suitable, and the testing would finish quickly. To my surprise, I found that the 3 on 2 Prusik did not perform as expected and just kept slipping at around 2kN.

I undertook one test on a 6 coil (3 on 3), and it slipped as well at 3kN so moved on to something else. I also came back on another day and tested another brand to get an idea if it was the cord I was using was causing some change in Prusik behaviour.

Note: The 3 on 2 Prusik is used (rather than 3 on 3) for practical reasons – the 2 coil part that’s next to the pulley tends to release well when being minded. In other words, it’s better to use.

Chatting with fellow rescuers and a manufacturer about the testing these are the questions/statements that came back:

  • How soft/hard was the 6 and 8mm cord?
  • But that’s not the brand we use.
  • But that’s new cord. Prusiks work better once they work in a bit.

Also, a few other things came to mind:

  • What is a suitable failure value for the load capture Prusik with new 6mm and 8mm cord?
  • What failure is critical: when the Prusik first slip or breaking point?
  • How many tests do we need to do to find out the testing variance you are naturally going to get?

Testing procedure

A repeatable test procedure was set up.

  • A 20t testbed used at the Chainman Ltd, 1 Cass Street, Sydenham, Christchurch.
  • Th cord used for testing was new.
  • All 8mm cord used a figure-8 knot on the bight at one end.
  • The 6mm cord tied as a loop with a rethreaded overhand bend (water knot).
  • 10cm tails used as standard on the knots and 4cm on the bends.
  • All knots and bends had hand tension.

Testing results

The 3 on 2 Prusik

Tying:
  • Tie a loop with the 6mm cord with an overhand rethread bend.
  • Thread the loop over the end of the 8mm cord.
  • Do a couple more wraps as you would tie a standard Prusik to get 5 coils (3 on 2)

Testing:

The 4 on 2 Prusik

To tie:
  • Tie with free ends in the 6mm cord.
  • Fold the cord in half.
  • With the bight, do two wraps to a get a 4 coil Prusik.
  • With one of the free ends tie two more wraps to 6 coils (4 on 2).
  • Tie a loop with the 6mm cord with an overhand rethread bend.
Testing:

And while we were testing – the Scaffold hitch

To tie:
  • Thread an 8cm length of 16mm webbing (with the ends sliced open) onto one end of the 8mm cord (as edge protection).
  • Tread the 8mm cord through the pulley becket.
  • Now tie a scaffold hitch – two wraps and thread back through the middle (as you would tie one side of a double fisherman’s).
  • Position the webbing and tighten the hitch up against the pulley becket.

Testing:

Summary

Q How soft/hard was the 6 and 8mm cord?
The PMI was a little softer than Edelrid which may have helped it grip a bit better.

Q But that’s not the brand I use.
A Change in brands did change the results.

Q But that’s new cord. Prusiks work better once they work in a bit.
A Yes and no. To a point – working in the Prusiks may improve performance. Prusiks can get dirty and stiff and not work as well. We should know what the failure values of new cord as a baseline.

What is a suitable value to fail at for the load capture brand new?
Considering we might put the load capture on the 3rd line or 4th line of a jigger (i.e. sharing the tension between 3 or 4 lines) I think a minimum failure should be 5kN. The system strength is, therefore, 3x 5kN = 15kN on the 3rd line or 4x 5kN = 20kN on the 4th line.

What failure is critical: when the Prusiks first slip or breaking point?
It’s good to know both values as well as the behaviour of the Prusiks. The Prusiks that reached the breaking point (sheath strips, Prusik cord breaks) all slid suddenly and then regripped successfully. There were some Prusiks where no further load could be applied (kept slipping).

How many tests do we need to do to find out the testing variance you are naturally going to get?
My goal was 5 on each brand and rigging style. I only did 3 and 4 on one. The 5-10 range would be ideal.

Final thoughts

  • With the testing we have done the 4 on 2 Prusik provides a much higher breaking point and less variable results.
  • Given we have watched all the testing live (and rewatched all the videos), we have the confidence in the 4 on 2 Prusik (instead of the 3 on 2).
  • We have rigged the 4 on 2 Prusik on a jigger system and hauled ourselves up and lowered down several times. It works well in a practical sense for load capturing and releasing.
  • It doesn’t mean to say you should use the 4 on 2. We encourage you to do your own testing especially for different brands of cord.
  • If you are using your jigger in an area of high load or potential high load, consider tying it off so that the load capture Prusik is not the only thing you are relying upon.
  • The Scaffold hitch (with webbing edge protection) is suitable to use as an 8mm cord termination on the pulley becket. It fails at around the same value as the figure-8 knot on the bight.

Get a download of all the Certificates of Testing with included graphs:

Working on the next post about the building a jigger. I will let you know when it’s done.

Grant

Grant Prattley
Grant Prattley

Instructor, Designer, Consultant, Guide, Author and Adventurer

at Over The Edge Rescue.

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