Author Topic: SLIDE AND GRIP HITCHES - Load testing to determine jamming threshold  (Read 3123 times)

agent_smith

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TEST REPORT
Knot specimens: Slide and grip hitches (symmetric and asymmetric) - formed from a 'round sling'

Test objective:
To determine the load threshold at which certain slide and grip hitches become difficult to release (ie the jamming threshold).
The scope if the testing had a very narrow focus - and that was aimed only at probing the load threshold where selected slide and grip hitches would no longer easily release their grip.
No testing exceeded 4.0kN and so the initial threshold jamming state was not reached.
This testing has practical application. A popular pre-rigged mechanical advantage system is the 'AZTEK' (made by Rock Exotica USA). This pre-rigged system employs a slide-and-grip hitch which functions as a 'progress capture device' (PCD). The AZTEK is reeved with 8.0mm cord and employs a 6.0mm diameter slide-and-grip hitch.
Link to AZTEK image: https://www.rockexotica.com/aztek/

Type of rope material:
Parent cord: EN564 Sterling 8.0mm 15.6kN MBS
Hitch cord: EN564 Sterling 6.0mm 8.8kN MBS
Ratio = 6/8 = 75%

Tester: Mark Gommers
Test date: 04 August 2018
Tester classification: Hobbyist / enthusiast tester

Test rig:
[ ] Dynafor 5 ton digital tension load cell
[ ] 2 ton lever hoist
[ ] Natural tree anchors in backyard
[ ] Unilateral setup - force generating machine (ie 'lever hoist') located on one side of knot - all force injected from one side
[ ] Lever hoist pumped by hand while observing load cell LCD display

Assumptions:
Slide and grip hitches work by 'crushing' their parent rope. As load increases, the degree of crushing also increases. An important characteristic of all slide-and-grip hitches is their ability to easily loosen when load is removed and then slide again. If the slide-and-grip hitch becomes seized, it will become difficult to manipulate.

Observations:
At loads up to 4.0kN, I observed no slippage of the slide-and-grip hitches. As load increased, the hitches began to deform and crush in on their parent rope. Despite a deformed appearance under increasing loads, this did not impair the ability of the hitch to hold position.
Upon removal of load after reaching a peak force of 4.0kN, I was not able to easily loosen and release the hitches so they could freely slide. However, no tools were required to loosen the hitches.

Conclusion:
It appears that beyond 4.0kN , none of the slide-and-grip hitches can easily be loosened to release their grip. The AZTEK is reeved with 4 'parts' and can be oriented to yield 4:1 M.A. or 5:1 M.A.
If the AZTEK is subjected to 16kN force, it will be divided evenly across all 4 'parts', with a resulting force of 4.0kN  on each individual part (4x4=16). Although, at 4.0kN load, the slide-and-grip hitches would not easily release.
This means that the real-world practical working limit of the AZTEK would be somewhat less than 16kN force.

It remains open to other testers to either confirm or refute these results (using EN564 certified cords of the same dimensions).
« Last Edit: November 27, 2018, 05:24:49 AM by agent_smith »