Saturday, 8 August 2009

Micro Wires - Soldered Brass Nuts v Swaged Aluminum Alloy Nuts

There was recently a discussion on UKC about small nuts. There are a few urban myths floating around about small climbing protection and this article was written to correct some of those myths.

Small soldered nuts are generally stronger than the equivalent size of swaged nut because you can get a wider diameter wire into the head of the nut, not because soldering creates a stronger join or because it removes a tight bend as the wire goes around the top of the nut (There is another apex at the carabiner loop). This advantage decreases as the head size increases.

When loaded to destruction these wires will normally break at the apex where they are being loaded i.e. at the carabiner. (Top tip: Use chunky biners with a wide rope/bar radius on crucial micro placements)

Thus use a 2.00mm wire and you can get circa 4-5kN out of a nut, but if you can use a 2.50 mm wire you can get 7 - 8kN.

This is where the IMPS/Brass Offsets/RP's win in sizes 1+ 2 - on an IMP/RP size 2 you can get a 2.00 mm wire in a miniscule head and achieve 5kn. A swaged nut of the same size (Wallnut 0) has to use 1.50 mm wire and the strength drops to 2+kN.

However jump up slightly in head size and you can get 2.00mm wire into a swaged head. The Wallnut 0.5 uses 2.00mm wire and is only very slightly wider than a IMP/RP 2 and is conservatively rated at 4kN. In a real world situation it is as/almost as strong as an IMP/RP 2

On IMPS/Brass Offsets/RP size 3 the advantage is further decreased because you can't get a 2.50 mm wire into the base of the head, but on the Wallnut 0.75 the head is now big enough to take a 2.50mm wire when swaged and also has plenty of metal to stop it pulling through placements. Thus the Wallnut 0.75 (same width as a IMP/RP 3) going forward will use 2.50mm wire and be conservatively rated 6kN - stronger than an IMP/RP 3.

There is a table of micro nut strengths/dimensions at in the Nuts and Slings section for fellow gear freaks.

It also needs to be remembered that the swaged micro nuts (i.e. DMM Micro Wallnuts) offer greater, surface contact which is an important consideration as a common mechanism for failure in real life is the nut pulling through the placement. Once again the devil is in the detail - using a harder (7075-T6) aluminium alloy nut as on the DMM Micro Wallnuts gives extra holding power compared to a softer 6000 series alloy, but is still (just about) soft enough to bite into the plavement.

This image shows what happens when the material used is too soft - the head just deforms under load and rips out. However make the material too hard and the nut won't bite into the placement and then runs the risk of being dislodged by quickdraw/rope movement.

Add to this the fact that standard steel wire as used on swaged nuts generally has a higher tensile strength than the stainless steel wire that must be used on soldered nuts.

On the other side of the coin IMPs/RPs and Brass Offsets will fit where the wider swaged alloy nuts won't go....

Each nut has its own strengths and weaknesses and the key thing is to understand them.

There is no perfect micro wire although an IMP/RP 2 comes pretty close; but a micro wire rack that includes IMP/RPs 2, 3 and 4, Brass Offsets 2, 3 and 4 plus Micro Wallnuts 0.5 and 0.75 is a really good starting point.

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