Breakable tensile member?

June 4, 2017 10:17 AM

Is there a such thing as something like a chain link that breaks at a known load? Like a shear pin, but for tension. Accidentally failing at too low a load is very preferable to not failing.

We are having some friendly marital disagreement about when it will be safe to hang a hammock from our two backyard palms. I think if I could put a breakable link in line which will save the trees in case of, e.g., hurricane, it would make everyone feel better, but my McMaster-fu is failing. I could do it with a shear pin but I'm wondering if there's something more directly applicable. Thanks in advance for any help!

posted by ftm to Home & Garden (13 answers total)

I don't know if they scale up, but this made me think of the safety collars my cats wear. Enough force and they simply unclip themselves.
posted by Emperor SnooKloze at 10:34 AM on June 4, 2017

Here you go.
posted by halogen at 10:47 AM on June 4, 2017

Yes, halogen's thing is what you need.

The wind load from a hammock pretty close to the ground is going to be a bunch less than the load from the tree canopies themselves in a hurricane. You also risk it breaking while someone's in it, which would be no fun.
posted by scruss at 10:54 AM on June 4, 2017

"Breakaway connector". For awhile, lots of ceiling fan pull chains had these, to keep the switch from breaking. (People complained anyway when they broke.)
posted by Huffy Puffy at 11:17 AM on June 4, 2017

The breakaway swivel looks like a nice piece of hardware, but when choosing a load rating, consider this.
posted by jon1270 at 11:40 AM on June 4, 2017

Man, it seems like such a thing would exist, but I can't find the right magic terms to find it either.

You could self-rig it with a tension spring and a pelican hook inline. Go tree - spring - hook (open end toward hammock) - hammock, then tie back the pelican hook release ring to the tree (or just to the non-moving part of the spring) with a string. If the spring stretches enough, the pelican hook will move away from the tree, which will cause the string to pull off the hook release.
posted by ctmf at 2:17 PM on June 4, 2017

You could probably do this with cable ties. Ties are rated at a specific breaking strength (50, 120, 250 pounds at the high end); you'd want to test them yourself (just hang weights off a loop) to see what safety factor is baked into that rating, then design a combination of ties accordingly. You need enough strength to support yourself with a safety margin, but fail at whatever wind load you think the hammock will transmit.

Bear in mind there may be some weather-related degradation over time, and there are dynamic loads as you get in/out of the hammock and if you swing; I suspect you're much more likely to have this fail with you in the hammock rather than fail as intended in a storm.
posted by Huffy Puffy at 2:25 PM on June 4, 2017

Why not unclip the hammock during a hurricane? Dynamic loads are no joke and I am very nervous your enjoyment of the hammock will be ruined by a broken coccyx when the link fails. #killjoyAlert
posted by gregglind at 4:22 PM on June 4, 2017

Screamers are used in climbing and activate at 2 kN. You might find them at REI (MEC in Canada), or a local climbing shop.
posted by Dashy at 6:02 PM on June 4, 2017

The "screamer" reminded me of the drag feature on a fishing reel. But you probably don't actually need the mechanism; you could use fishing line. Know your strength and you know how many loops to use.
posted by Huffy Puffy at 6:19 PM on June 4, 2017

Gliders use "Weak Links" when being towed from an airplane to ensure they can't be damaged by a violent maneuvre under tow. They aren't easy to find, but you can get some here.
posted by Popular Ethics at 6:38 PM on June 4, 2017

I'm in a hurricane zone, and I have to admit that I'd be truly amazed if a hurricane ripped that it would be the palm tree that fails before the hammock being ripped to shreds (enough to not catch the wind with much gusto anymore). If a high CAT storm is coming through, you're hopefully already cutting trees and such back to give them a fighting chance away, so just unhook the hammock then (and that's more to save buying a new hammock and being a good neighbour then anything else)

If it is a major concern for your family, why not just use some known tensile strength climbing ropes to tie the hammock up to the palm tree? Basic paracord is rated to 600lb or so, right? I'd like to assume that should cover most people jumping on the hammock-- though I'm in gregglind's camp, I'd rather overkill on my fasteners and save a fractured bone then try and engineer it to a precise "greater then the jolt from a falling human but less then a jolt from a 135+mph wind gust" tensile strength.

Of course, you could aways just get a rope-tied hammock, not much for the wind to catch on them.
posted by Static Vagabond at 7:03 PM on June 4, 2017

Thanks for all the help & ideas! To assuage some worries about my tailbone and/or the trees, this would be more along the lines of a moral compromise but I always like to consider engineering solutions. Thanks again!
posted by ftm at 10:40 AM on June 5, 2017