Favourite knot(s) for connecting 2 ropes for heavy loading

[Jeff]

I was taught the alpine butterfly bend as one that can always be easily untied after loading, but I'd put it in the category of one that can usually be untied easily after loading. MUST be properly dressed and set!

 

[Howie65]

It is one in a family of knots based on interlocking overhand knots and in this group one that jams less compared to the Ashley and Hunter's. But for extreme heavy load as it's the case for rigging is seems that other knots are a better choice.

Due to all Ideas collected in this thread I would try now a double sheet bend whereas the second loop is slip-tied (correct english?). Maybe more loops before the slip give a even better result (more friction and the slip can be open more easily and this is like the removed sprig). Unfortunately I cannot test it because my wife has only 117 pounds (ok together we have 335 but I have to pull the rope).

Who can test this with 20 tons?

P.S.: If the greenhorn (me) is annoying with all his input than please give me a note . Now it's 23:46 in germany and I will continue with my movie.

 

[Reliable tree man]

Shucks, would be nice to see the welded knot!

But I'll step in for Agent_Smith --who asked the pertinent
question--, and confirm that you show the right knot.
(Sometimes the knot is mis-presented, with the two ends
turning in opposite directions, like the butterfly or #1425a.)

Do you have any hunch as to why you had a problem this time,
other than possibly simply more load on the line? --maybe different
line (more elastic, say)? For this knot is regarded as pretty jam-proof.

There are a couple of like knots presented in Ashley's Book of Knots
--viz., #1452, #1408, #1425-- that have interlocking overhand components
and which should be jam resistant; #1408 could be regarded as a symmetric
butterfly knot.

*kN*

The two lines that were connected were samson arbormaster climbing lines. (not being used for climbing) Not my choice for most rigging applications but was all my employer had.

These lines are pretty stretchy which is what I am thinking was the cause of the unbreakable knot.

My go to for this task is typically two bowlines or double bowlines on a bight which have proven to be bomb proof.