How strong are those dinky 3/16" quicklinks? Like anything else, the answer depends on where you get them. The attached image shows the results of just one test to > 3280 lbs, where the 10.2 mm dynamic rope broke. I used my jeep to provide the force, and measured the force with a 3-ton capacity crane scale. The crane scale has a time constant of about 1 second at best, so it generally won’t catch the peak force in a dynamic break test. The dynamic rope was connected to the quicklink, and to the jeep, with figure-8s-on bights, and that’s where the rope broke.
The movie of the test is here.
Where I note the rope broke, the wrong end is
visible; I was holding the far end (which had been attached to the
jeep) in my other hand, but that is out of camera.
Quicklinks normally fail at low stress when an improperly machined
female nut slips off the male thread. In the tests I've seen, where
quicklinks failed well below rated strength, the tester had typically
bought a boxful of anonymous links on ebay, and the poorly-fitting
threads (which were obvious at the start) deformed and pulled off at
much less than 1000 lbs force.
I bought quicklinks from US Stainless, National Hardware (cheapest) and
French Maillon Rapide (which means “quick link”) made by Peguet, and
sacrificed 3 from each set of 10 for testing to 1000 lbs;. After
tensioning, I was able to unscrew each one by hand or with slight
wrench force.
The bad thing about small quicklinks, is that they put more strain on
the rope, than much thicker links. The tight curve radius reduces rope
strength, and cinches the rope around the metal so force doesn't
equalize between strands in a quick pull.
I’ve watched youtube videos of pull tests on small quicklinks. I note
that the EN standard for testing quicklinks requires pull by 12 mm
diameter round pins on each end, whereas some tests used <8mm
diameter pins (actually other quicklinks). A metal quicklink is
more likely to break when pulled by another small metal object, which
basically applies a point force – especially a metal object with low
radius. In contrast my test involved soft, deformable ropes on
opposing sides of the pull.
These results should not be too surprising. 10.2 mm dynamic ropes are
not generally tested for breaking strength; they are tested via the
UIAA protocol for multiple drops over an edge (simulating the last
piece of protection) with an 80 kg weight. However, in straight pull
tests, they do break at about 5000 lbs when there are no knots. A
figure 8 reduces the strength to about 65%, so 3280 lbs is quite
reasonable.