Yat technical question

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Joined: Sat Jun 12, 2004 5:43 pm

Yat technical question

Post by JohnMac » Sat Mar 21, 2020 6:06 am

We need some help! Anybody into mechanical engineering, possibly at uni? At the Yat we have long time experienced success with 8 plait cord:
English Braids, 4mm, K40, breaking strain 320kg. Expensive but wears well, lasting and easy on the hands. With floods now another metre higher we need to install a break point in the cord. Can anybody support us in a solution to this challenge? Access to a machine that measures breaking tensions would be a great help.

Steve Agar
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Joined: Wed Jun 17, 2015 9:30 am

Re: Yat technical question

Post by Steve Agar » Sat Mar 21, 2020 10:42 am


Making a weak link with a known break load is pretty straightforward if the material (preferably stainless steel) is purchased with its mill certificate that provides the tested breaking stress. Similarly, bolts (particularly high grade socket head screws) can usually be purchased with a test cert., and can be used as the weak point in a coupling. Even without a test certificate, an A2 or A4 stainless bolt will have a pretty consistent break load in shear - good enough to design a weak link that will break before the line does. One problem is that all ropes degrade with time and exposure, so the stated 320kg strength really ought to be tested annually using a sample of used rope. As you say, this is an ideal uni student project, as most mech. eng departments will have access to a suitable test machine.


Mike Mitchell
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Re: Yat technical question

Post by Mike Mitchell » Sun Mar 22, 2020 9:21 pm

Did some tests with first a 3mm cord loop in the system using 4mm cord for the main line.
With a 6x pulley system and all the force I could get on it, I could't brake it.
So changing the system to having a single 3mm Cord section in, then the 6x pulley system snapped it.
How much force, not a clue but being a Climber and hauling a 70kg Bag up El Cap it was a lot stronger that I would have imagined.
Going to see I can get some Video on Facebook, Canoe Slalom UK.

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Location: Pinkston

Re: Yat technical question

Post by JimW » Mon Mar 23, 2020 3:42 pm

Your problem with rope and cord is that it is generally rated by minimum guaranteed breaking load (un knotted), an individual sample could have an actual breaking load much greater than that, at least when new and before degraded by UV and possibly water (nylon for example has a relatively high water absorption rate which can weaken it significantly, but it is not a common material for ropes).

It would require a lot of extra rope, but have you considered running your main bearer over a pulley or through a ring on the island and then back to a higher point on your post so you could manually cut it at the footpath side and haul in on the remaining line so it unthreads on the island and all comes back to you. I'm assuming you can get down there to recover the kit before it compeltely floods but after it wouldl be too dangerous to try and paddle to the island (strainers where the island used to be) to untie it.
Entanglement willing of course, I have let enough kite lines go under tension to realise you may just get a birds nest of the cut end around the pulley...

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