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Public Lab Research note

KAP'n Hook fail

by cfastie | March 30, 2014 01:41 30 Mar 01:41 | #10244 | #10244

Peter Bults of KAPshop generously sent me a message when he saw the 3D printed KAP/BAP rigs I started to introduce this week. He was quite confident that my KAP'n Hooks that attach the Picavet to the flying line would not survive the line tension produced by a hard pulling kite. I hadn't thought much about the forces produced by the kite line, but I immediately started to.

If the kite line is wrapped in a continuous spiral around the hook, line tension could cause the whole hook to bend (above) and also exert more local bending forces near the ends.

These forces don't have much leverage on the hook, so the forces have to be very strong to deform it. But kites can pull very strongly.

If the line spirals in opposite directions on the two ends of the hook, there is pressure to bend the hook sideways. As always, there is also local pressure near the ends of the hook.

I did a crude test to see how much line tension the KAP'n Hooks could withstand. I attached some 200 pound braided Dacron kite line to a hand winch and strung everything between two trees. I wrapped the line around a KAP'n Hook and cranked down on the line. I didn't have any way to measure the line pull, but it was not hard to break hooks printed in PLA (polylactic acid). They just snapped. Hooks printed in ABS bent a little before they broke.

In the poor quality video below (I needed 600 frames per second), the orange and purple hooks are PLA, and the black hook is ABS.

So Peter was right, a tight line applies substantial force to anything it is wrapped around. These 3D printed hooks would work fine on balloon flights or slack line kite flights, but if the wind picks up, they would not be very reliable. I will have to find an alternative to use with the 3D printed Picavet. Thanks, Peter, for probably averting some disasters.

Can anyone suggest a design or product that could function as a line hook and cost less than a dollar?


we now ship the smallest aluminum mini-carabiners with our balloon kit. They work great. We can't sell them for less than $1, but a dozen cost $5.

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Chris,you mentioned these in your email about my order. Brooxes Hangups™, are a more expensive but proven solution at $3.50 a pair, from http://www.brooxes.com/newsite/BBKK/BBKK-PARTS.html. The listing reads, "These simple, strong, lightweight aluminum hangers make attaching the Picavet rig to the kite line a simple matter. Hangups lace right into the Picavet lines, and are rustproof, foolproof, quick and easy to attach to the kite line, and quite secure in a wide wind range."

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Pat, I've used Hangups a lot and they are probably the best solution. Brooxes KAP-klips are also proven effective, and may be easier to use. They are a modified version of long-line hooks which commercial fisherman use to attach countless baited hooks to a single long fishing line. The standard long-line hook is five inches long, but I found some three inch ones which should be perfect. They don't cost any less than Brooxes KAP-klips ($2 per pair) so KAP-klips may be the way to go. I also ordered some keychain carabiners to subject to my stress test. I also got a spring balance so I will know how much pull I am subjecting them to. Can't wait.

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Have you tried our FABAhangout http://old.publiclab.org/sites/default/files/FABAhangup.png yet? Yesterday I tested a simple and cheap alternative using 2.4mm galvanized wire (for tensioning metal fences) as you can see here: https://www.flickr.com/photos/fabain/13701204823/sizes/o/in/photostream/

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Your bed spring hangups are a good idea. I found these for sale, but even smaller ones would be better. I might have to go looking for old beds.

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