Author Topic: Looking for a knot  (Read 11686 times)

jglohr

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Re: Looking for a knot
« Reply #15 on: June 14, 2006, 07:11:25 PM »
Thanks Derick,

That looks like the exact solution I need to my problem. I'll give that a try, and post the results.

Josh

jglohr

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Re: Looking for a knot
« Reply #16 on: June 14, 2006, 09:50:13 PM »
I just got done testing the inline method of tying the knot. At first it I thought it would work perfectly. But it actually did the same thing as before - the knot slid to one side. After thinking about it more I understand why.

Here is a diagram showing the forces acting on the rope:



There are two possibilities for the knot to move when tightened:
1) the red end slides through hitch clockwise, subsequently tightening the loop, while the hitch remains fixed (desired case)
2) the hitch slides counterclockwise along the red end. The red end and entire loop remains completetly fixed.

In order for case 1 to occur, F_friction hem must be less than F_friction knot. Because F_friction hem is a huge force when any tension is applied, the entire loop essentially is locked in place. Thus, the force applied can only overcome F_friction knot, which makes the hitch slide instead of the loop (case 2).

In most other applications, this adaptation of the knot would work as desired, because there wouldn't be a frictional force tangential to the loop. That is why it works with a free rope.

All was not lost, however, as this method is still better than the method I was using. If I pull a little bit, and pull only on the hitch end, I can work the knot back over to where it should be. Using this method of applying a load, adjusting, and applying a load again, I can achieve the results I am looking for.

This has been a good learning experience. Thanks for all the interesting posts.

Josh

jglohr

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Re: Looking for a knot
« Reply #17 on: June 14, 2006, 11:19:17 PM »
Now, my question is, which of the multitude of slip/grip knots is the easiest to untie/loosen when in tension? This needs to be adjustable, which means easy to loosen as well. The adjustable hitch grip was pretty easy to untie in the non-tangent setup, but is rock solid in this setup.

jglohr

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Re: Looking for a knot
« Reply #18 on: June 14, 2006, 11:46:05 PM »
After looking at me previous post I started thinking more on the forces. Now I think I was wrong - for both cases to occur the frictional forces in the hem would act against each end.

Now when I look at it I think you are right Derick. It should stay centered. Yet it doesn't.

I need to think about this one some more.

jglohr

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Re: Looking for a knot
« Reply #19 on: June 14, 2006, 11:54:44 PM »
Ok, let's try this again.

If I am thinking about it right now, F_friction hem is not acting as shown in my diagram, but in fact acting in the opposite direction in the bottom half of the circle (assuming when tensioning, there will be a fixed point exactly 180 degress from the knot, and on the bottom half, the rope will be stretching counterclockwise from this point, and on the top half clockwise. Thus, both case 1 and 2 must be happening equally, as the forces are balanced and equal. Thus, while the ideal case 1 is happening, case 2 is also equally happening, resulting in some movement of the knot.

Let me know if this makes sense, or if my logic is wrong.

Willeke

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Re: Looking for a knot
« Reply #20 on: June 15, 2006, 12:47:45 AM »
Josh,
as a member you can modify your old posts. That way you do not have to have 2 or more posts telling what you have missed or done wrong in a previous post.

(You are welcome to post as often as you feel needed but when reading I feld you were not happy with the posts.)


Willeke
"Never underestimate what a simple person can do with clever tools,
nor what a clever person can do with simple tools." - Ian Fieggen

Writer of A booklet on lanyards, available from IGKT supplies.

DerekSmith

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Re: Looking for a knot
« Reply #21 on: June 15, 2006, 12:51:27 AM »
Of course Josh, you are right.

If you fix the opposite point of the cable to the hem (effectively achieved by the friction in the hem), then as you pull up the loop, the circle gets smaller.  So BOTH pieces of cord become too long and move away from the desired point.  Thats not a problem for the red end, as it is all the same, but the knot end effectively moves forward because the piece of cord from the half way point to the knot is now too long for the reduced size half circle.

Back to the drawing board - sorry.

Dan_Lehman

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Re: Looking for a knot
« Reply #22 on: June 15, 2006, 01:13:20 AM »
With Derek's general set-up--eye in end of hitch, oriented to pull tangentially--,
one should be able to control where the hitch is, by holding in place this eye
as the hitched part is drawn through the knot, or pulling/releasing the eye to
whichever direction seems needed.  However, so long as predominantly the line
is drawn through a stationary knot, that line is moving ONE way through the
entire and increasingly tight-upon-it hem, and will drawn the hem with it.
Maybe there is a strong force, thus, for the hem to arrest this drawn-end movement
and coerce compensating movement of the knot, though I'd think that one could
position one's hold on this so as to at least become aware when this force occurred.

But one cannot have the two ends (i.e., knotted end & hitched end) make
equal movements in opposite directions (so as to equalize the line's draw through
the halves of the hem), because the knot is around the other line and would
have to move into the hem whatever distance was needed to balance.

--------
There is a (theoretical  ;) ) way to, er, *centralize* things, so try this tactic:
1) size your cord so that it wraps around the desired circumference TWICE and more;
2) form a Prusik Hitch around air in the center, and bring each end, in opposite
direcitons through the hem, around through this hitch;
3) work the hitch snug/tight;
4) pull ends in opposite ways to tighten evenly (in theory--that's all I do here!)

In this set-up, having the doubled cord through the hem might help in some
way diminish the friction against it?  In any case, the cord will be moving equally
in both directions, so the hem should be *unruffled*.

As for ease of untying, well, c'mon, ain't 2 outta 3 okay?  Is this g'mnt work?
One could get extra clever (pain on the tying end) and arrange to make this set-up
in reverse, beginning with a loop to cross the ends (which loop will be enlarged
in pulling it wide to tighten...), run ends through hem and each emerges to tie
one half of the Prusik, being themselves tied off (Reef Bend) to form the Prusik's
cross-part (I've seen it recommended to orient the typical Grapevine bend
in a climber's/caver's "prusik sling" positioned here so as to keep the knot from
jamming so much (YMMV))!  That's maybe too clever by half, but, hey, it's only
keystrokes for me!   ;D

The above, yes, entails more cord.
But one expects to suffer the usage of this, so if it's a better mousetrap in that,
it might be worth it.  (Btw, the idea of cost of these fiddly bits being relevant
surprises me--it just isn't all that much cordage!)

--dl*
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« Last Edit: June 15, 2006, 01:17:00 AM by Dan_Lehman »

jglohr

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Re: Looking for a knot
« Reply #23 on: June 15, 2006, 02:53:26 AM »
I believe it is in fact possible for both ends to move opposite. Both ends are being pulled with equal force, which means that, assuming frictional forces to be equal around the entire hem, deflection should be equal on both sides of the "equator" (a line drawn from the knot to a point 180 degrees). On the non hitch end, this deflection is achieved by sliding through the hitch. On the hitch end, because the force is directly pulling the hitch, the deflection is taken up by the hitch moving, and it is moving directly into the hem. Which, of course, is the whole problem, because it stretches the material eventually to the point of tearing.

Another way of thinking about this is, that the hitch is not in fact stationary as previously described - the force is directly pulling it into the hem.

All this is assuming the drum isn't fixed. If I hold the drum in my hand and vary the angles of the pull force I can effectivly make the force not equal and oposite (IE, hold the drum tightly and pull only the hitch end, but in the opposite direction to pull it back after it has moved. I think this might be what you were saying Dan. Another option may be to put a nail or some hardware as a stop for the hitch, although given the low strength of mdf I'm not sure this would work.

I am intrigued by the Prusik idea, my one question is, what keeps tension on the hitch? Being independent of the tension in the main line, there is nothing keeping it from coming undone. This line of thinking is what I was originally looking at.

Yes, cost really isn't that big of an issue, other than that I eventually plan to make 12 of these things for an entire set. I just like to design my projects as cheap as possible, because DIY projects not planned out correctly almost always end up costing me more than buying the thing new. Now I notice that PP isn't really much cheaper than Nylon (at least not at the store I was at).

Dan_Lehman

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Re: Looking for a knot
« Reply #24 on: June 15, 2006, 09:23:23 AM »
Quote
Another way of thinking about this is, that the hitch is not in fact stationary as previously described - the force is directly pulling it into the hem.

Well, again, one should be able to simply hold the end-eye of Derek's knot so
that the hitch doesn't move with the end and thus the end is drawn through
the hitch, whose only movement should be a bit inward along the radian--you
again are holding it against moving with the draw of the end through it,
but not pulling away from that draw!

Quote
I am intrigued by the Prusik idea, my one question is, what keeps tension on the hitch?

The two ends (well, two LOADED parts--no ends are "loose" here) run in opposite
directions around the drum, passing head-on at 180deg point, continue over
each other's outset to re-meet at ground zero (the knot) and dive through it
still in opp.directions.  Pulling on these ends pulls around to tighten the knot.
(Though one will want to set the knot with some pre-tensioning, to draw down
the wraps.)  The tensioning of the ends loads the hitch.
In the "too-clever-by-half" way, presented as one way to have things easily
untied, you tie this set-up in reverse, essentially having the first set-up
with ends welded together after passing through the hitch, then cutting the
Prusik's *back*, and then tying that cut back (which spans the wraps).
Otherwise, untying will entail some bit of gripping each end and pushing on
the Prusik to draw out the end.  Maybe pliers.  You'll be best to see what sort
of tension & tightness arises from the materials and need.

(-;

jglohr

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Re: Looking for a knot
« Reply #25 on: June 15, 2006, 10:59:39 PM »
That's the whole problem, it is impossible to just hold the hitch without drawing it away. By holding it, and applying a force on the other end, you are applying an equal and opposite force on the end you're holding. Because there always must be a constraint with an equal and opposite reaction to remain statically balanced, and in this case it is the hitch end, the only way to avoid this is to hold the drum, so the drum essentially becomes a contraint. I don't like this option because the drum isn't that strong.

I understand the Prusik idea now, and tried it out on a rope outside the drum. I think it is the closest to what I am looking for in terms of equall draw on both ends. I will sew up a drumhead and try it out.

DerekSmith

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Re: Looking for a knot
« Reply #26 on: June 28, 2006, 09:39:52 AM »
Hi Josh,

I don't know if this issue is dead yet, but it has been bugging me since the realisation that both ends of the drum loop move out of the seam equally when you apply tension.  To achieve the goal you set, both ends would have to move through the knot as tension is applied.  Also, you wanted a means to slacken off the tension, so the knot really had to be a good slip/grip hitch.

The nearest I have come up with so far is to tie the working knot(s) using a second cord so that both of the tensioning ends can flow in or out of the seam without the knot moving.

In the following picture, the drum tensioning cord is the white loop with ends C and D.  You can use the overhead support and tensioning footloop as in the first proposal to apply tension to the ring.

In the red cord, I have tied two simple four turn slip/grip hitches in a back to back configuration.  In use you will have to fix the ends A and B in order to stop the two knots moving towards oneanother as tension is applied.  Alternately, have a helping hand hold the two ends in tension while you put tension onto C and D to tension the drum.  I found a simplesolution was to tie A and B respectively to the ends of a short length of cane to keep them in place.

Slackening the knot is easy.  Simply retension C and D to take the tension off of the hitches, pinch the two hitch 'noses' together and then relax the tension in C / D.  The white cord will then slip through the hitches until you stop pinching them together.



Here is a closeup of the knot 'assembly'



In practice you will need to tie the two knots as close to oneanother as possible.  I have left it rather long in this example in order to be able to show the layout.

Can anyone think how we could incorporate both these SG hitches into one knot?

Derek

Dan_Lehman

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Re: Looking for a knot
« Reply #27 on: June 28, 2006, 07:21:14 PM »
Can anyone think how we could incorporate both these SG hitches into one knot?
Derek
Put a Half-hitch between the knots, to hold the opposed draw-string ends together
when tensioning!?

--dl*
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jhonny3

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Re: Looking for a knot
« Reply #28 on: July 10, 2006, 12:10:16 AM »
Not sure if this will help...though the hem might be a bit of a problem, but you could tie two inline adjustable hitches, essentially making a water knot but with adjustable hitches.  I don't have either reference or digital camera to make heads or tales of this, but I'll try to better describe it as follows.  Same as the Water knot, but instead of overhand knots, tie adjustable hitches.  The hitches allow them to be moved from the initial point and still hold. I've found it holds very well, indeed, I've never seen it slip, the biggest problems with this solution lie in working the hitches through the hem and the rope doubling when you move the hitches.  Hope it helps, and if you still need clarification feel free to ask :D

EDIT: OK, so I missed the second page of the forums the first time, so this is veerrryyy close to DerekSmith's idea, only instead of using a second piece of rope, just use the same rope.  Now that I look at it, Derek's might be better...try em both and see how they work! ;)  Also, now looking at the original post of mine, I realised that in my tired state I said water knot...I meant fisherman's bend...um...really! ;)
« Last Edit: July 13, 2006, 04:09:03 AM by jhonny3 »