Author Topic: Hunting for a knot  (Read 7063 times)

coyoteboy

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Hunting for a knot
« on: January 16, 2013, 04:40:48 PM »
Dear all,

Please excuse my rather rude entry into this world but I'm a long time user of basic knots but have never been drawn into the world of knots, that I wasn't even aware existed until now!

I'm hoping you can help me with what will probably be a really easy quest, I've been to a few sites like the animated knot site which were quite helpful but I need to have a "search by characteristics" option as I don't know names. I'm looking for a knot to tie two ends of a loop together while being self holding and sort of constricting. The application is a bit unique but imagine using a piece of fishing line to loop through an anchor, come up through 2 holes in a board and tie it down. The board is spring loaded so any knot should be constricting. The current solution is to tie a half hitch, hold the half hitch with tweezers, then try to tie a secondary half hitch over the top. This works, but it tends to a) Loosen the first knot a little when tightening the second knot and b) slip once tightened because of the material.

I have no scope for changing the material and I need to maximise it's strength when knotted. If I could find (or rather if you could help me find!) a knot where I could tie the two ends and then just keep pulling to pull the board towards the anchor and tighten further and further like a ratchet strap I'd be really grateful. I'm not worried about un-tying the knot (the line is disposable after use).

Cheers,

James

roo

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Re: Hunting for a knot
« Reply #1 on: January 16, 2013, 05:39:17 PM »
Dear all,

Please excuse my rather rude entry into this world but I'm a long time user of basic knots but have never been drawn into the world of knots, that I wasn't even aware existed until now!

I'm hoping you can help me with what will probably be a really easy quest, I've been to a few sites like the animated knot site which were quite helpful but I need to have a "search by characteristics" option as I don't know names. I'm looking for a knot to tie two ends of a loop together while being self holding and sort of constricting. The application is a bit unique but imagine using a piece of fishing line to loop through an anchor, come up through 2 holes in a board and tie it down. The board is spring loaded so any knot should be constricting. The current solution is to tie a half hitch, hold the half hitch with tweezers, then try to tie a secondary half hitch over the top. This works, but it tends to a) Loosen the first knot a little when tightening the second knot and b) slip once tightened because of the material.

I have no scope for changing the material and I need to maximise it's strength when knotted. If I could find (or rather if you could help me find!) a knot where I could tie the two ends and then just keep pulling to pull the board towards the anchor and tighten further and further like a ratchet strap I'd be really grateful. I'm not worried about un-tying the knot (the line is disposable after use).

Cheers,

James
I admit, I'm not sure I can visualize what is going on, but if you need constriction in fishing line,  I'd have you look at a Slippery 8 Loop to see if there is any way you could fit in your setup:

http://notableknotindex.webs.com/slippery8.html

It may involve linking one Slippery 8 Loop to another Slippery 8 Loop, but then again, you may not have room for that.

There's another thread that may be pertinent to this:
http://igkt.net/sm/index.php?topic=3659.msg21157#msg21157
http://igkt.net/sm/index.php?topic=3659.msg21153#msg21153
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coyoteboy

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Re: Hunting for a knot
« Reply #2 on: January 16, 2013, 06:21:00 PM »
Hmmm I'm going to go and experiment with some of the suggestions from the threads you mentioned, thanks - they look really helpful. For clarity, here's the situation (attached).

The green panel is always trying to move upward away from the black panel and anchor point, because it is spring loaded, we need to tie it down at maximum spring compression (the green panel hits some stops at maximum spring compression). The thread is passed through the anchor point, then up through two holes in the green panel while holding the green panel in place against it's stops. At this stage we normally tie a half-hitch and then attempt to hold the panel down AND maintain tension on the knot while tying a second to lock it off. Any slip in the first knot leads to a 1-2mm slip in the knot point, which allows the panel to spring up off it's stops, which is not ideal.
As I think you may have guessed from my cryptic post, what I'm looking for is a knot that will be easyish to tie in a thin line and while you are tying it, the process of tightening the knot also tightens the loop (pulling the green panel downward, locking the pair together neatly). This knot, obviously(?), must not come undone on it's own or with the force of the spring pushing the green panel up.

The problem boils down to effectively the same as tying your shoe lace with a knot that ever-constricts your foot as you pull on it, never letting go.

I've tried a couple of bends but they tend to slip with this material. Maybe a combination will be the cure.
« Last Edit: January 16, 2013, 07:15:25 PM by coyoteboy »

roo

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Re: Hunting for a knot
« Reply #3 on: January 16, 2013, 07:29:51 PM »
The problem boils down to effectively the same as tying your shoe lace with a knot that ever-constricts your foot as you pull on it, never letting go.

I've tried a couple of bends but they tend to slip with this material. Maybe a combination will be the cure.

A Versatackle may also work if you have the room:

http://notableknotindex.webs.com/Versatackle.html

You could just use overhand loops to make it if the line is disposable.

By the way, how wide is your blue rectangle in your diagram (left to right)?
« Last Edit: January 16, 2013, 07:31:45 PM by roo »
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coyoteboy

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Re: Hunting for a knot
« Reply #4 on: January 17, 2013, 10:10:16 AM »
Blue rectangle is 15mm across the base and top and 4mm up the sides, meaning tiny slips cause me to lose all tension.

Cheers, will give some more a go today - people are wondering why I have a line of chords tied to a pen on my desk :)

roo

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Re: Hunting for a knot
« Reply #5 on: January 17, 2013, 04:22:12 PM »
Blue rectangle is 15mm across the base and top and 4mm up the sides, meaning tiny slips cause me to lose all tension.

Cheers, will give some more a go today - people are wondering why I have a line of chords tied to a pen on my desk :)
Wow, that is small.  I guess that rules out anything that requires much space. 
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SS369

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Re: Hunting for a knot
« Reply #6 on: January 17, 2013, 04:31:48 PM »
Hello James.
I've got a question. Is there room, hole diameters sufficient, to allow for multiple passes of the string around the objects?
If so, then I would do this after compressing the affair and I suspect there will be enough tension retained overall to tie the lock off knot.

Hope this helps.

SS

Dan_Lehman

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Re: Hunting for a knot
« Reply #7 on: January 17, 2013, 05:29:24 PM »
Hello James.
I've got a question. Is there room, hole diameters sufficient,
to allow for multiple passes of the string around the objects?

Good question!  One can think of trying to implement the
sort of binding structure that I presented in an old thread
about "binding knots" (or some such title), in which one
creates interlocking opposed bights that nip the tails ever
tighter as the structure is tightened.  This well suits this
problem statement, but you sure have a small work area!
(And one can wonder at how effectively my structure will
hold in this case of small quarters & nylon monofilament? line.)


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coyoteboy

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Re: Hunting for a knot
« Reply #8 on: January 17, 2013, 06:48:36 PM »
roo - It is small, but the thread is only 0.2mm diameter :) It's very much a surgical precision thing going on!

SS369 - room, yes, possibility of it happening - no. The string ultimately has to be a single thread (it's getting cut by a device in the anchor point, adding another loop would cause issues).


SS369

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Re: Hunting for a knot
« Reply #9 on: January 17, 2013, 08:01:15 PM »
Your comment, "It's very much a surgical precision thing going on!" brings to mind a thread > http://igkt.net/sm/index.php?topic=4107.0 which has additional links and some excellent photos (by a forum member) of tied examples.

Food for thought at least.
Please let us know if any of these is a winner.

SS

Mike in MD

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Re: Hunting for a knot
« Reply #10 on: January 18, 2013, 03:14:40 PM »
I_ll try to add my two cents. 

The blue rectangle is 15 mm on top, so I will estimate that the green plate is 45 mm by 45 mm.  You can have only a single thread through the anchor device at the black plate. 

Can you put more structure on the top green plate?  I envision two small posts and, for finishing, a cleat or small screw clamp.

Definitions: I will call the holes in the green plate A and B.  The perpendicular bisector of line segment AB divides green plate in two: side A includes hole A, same for B.  There will be a post associated with hole A (called post A); same with B. 

For working space, I want post B as far from hole B as I can get.  So I would put post B on side A, and post A on side B.  Hopefully, this would provide about 25 mm or more between hole and post. 

I would take the line coming out of hole A, make a loop or noose (my choice would be a Double Dragon), and slip the loop over post A.

Then I would take the line coming out of hole B and start a trucker_s hitch with a loop close to hole B.  There has been a lot of discussion here about the proper loop.  I would use a Bell Ringer_s with a half-hitch lock.  The Butterfly is a good possibility.  I don_t know what it is like to work with 0.2 mm rope.  After making this loop, take the line around post B, which is about 25 mm from hole B, back through the Bell Ringer loop, push down on the green plate, and take up the slack on the trucker_s hitch.  Finish by wrapping the working end about the cleat, or lock with the small screw clamp. 

You should review a picture or video of the trucker_s hitch to see how it works mechanically.

What do you think?
Mike
« Last Edit: January 18, 2013, 03:21:55 PM by Mike in MD »

coyoteboy

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Re: Hunting for a knot
« Reply #11 on: January 23, 2013, 01:20:54 PM »
The versatackle is currently winning the race purely because of it's action but it's posing a problem in tying - getting the two loops up onto the top of the green panel with exactly the right distance between their tips, after tensioning and tightening is proving tricky! I'm also looking at loop sizes of approximately 2mm circumference so I'm building it with tweezers and a small vice!

roo

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Re: Hunting for a knot
« Reply #12 on: January 23, 2013, 04:14:14 PM »
The versatackle is currently winning the race purely because of it's action but it's posing a problem in tying - getting the two loops up onto the top of the green panel with exactly the right distance between their tips, after tensioning and tightening is proving tricky! I'm also looking at loop sizes of approximately 2mm circumference so I'm building it with tweezers and a small vice!
Just out of curiosity and if it's not too much of a secret, what are these things that you're fastening together? 
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SS369

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coyoteboy

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Re: Hunting for a knot
« Reply #14 on: January 24, 2013, 05:20:06 PM »
Sure, a little more info is to point out that my knot search is for reasons of developing improved designs of PCB-based deployable solar panels for nanosatellites (Satellites in the region of 1-3Kg, usually educational or technology demonstrators that hitch a ride with larger ones). The Dyneema monofil is used to hold the deployable solar panel shut against the body mounted panel, and is then subsequently melted by a thermal knife circuit to release the panels when in orbit (It's a fairly standard practice in concept, but in practice there's few fixed methods).

Some key problems inherent with it are the panels are relatively heavy for a fine filament thread and very expensive, the threads MUST release with redundancy (hence the inability to multi-wrap) and any leftover parts must be bonded to the craft so they don't float in front of lenses etc. The whole system undergoes vibration testing and significant vibration at launch, where the panels and cord can see loads tens of times higher than normal static loading in a large range of frequencies. If the tie down slips, the solar panels deploy within the launch structure, damaging the panels and jamming in the launch vehicle. Also, the total height envelope for both PCBs, cells, hinges and tie down/release mechanisms is 6.5mm.

So far a few knot types have been tested in full vibration testing and we're confident with their performance but I'm looking for a ratcheting knot to make assembly easier to pre-load the dyneema (and hence the panels) without sacrificing locking potential.