searching for a self-tightening, locking knot for this application -attached img

[SS369]

Hello knothor and welcome.

If you are considering the modification of adding a post/bollard then you can consider many hitches. One post would do the trick. Start and end on the one.
Perhaps the Lighterman's Hitch. http://www.animatedknots.com/lightermans/
Or a many wrap affair such as a Tensionless Hitch with a suitable tie off.

SS

[knothor]

yes I don't have a post option in the current design but I need to have two of these contraptions anyway so the next iteration will have whatever this one lacks. And I will try to somehow replicate those changes on this first prototype. Anyway I think plastic pins  that have a tubular shape so they can accommodate two short nails (sort of like the guitar bridge pins).

I attached a quick sketch.

Now... what knots are you suggesting? This would be again a mid-air sort of knot and I am ok with backup knots.

Edit: sorry @SS369, I only now realized there is a second page opened for this thread . How would I do about using the actual bolt as a pin/post/ballard (many names ). My ends of the string are visible in the first picture. I see some logic in releasing the knot of some stress by using two pins as suggested by @alpineer but I see the following scenario for one-post solution: I tie one knot with some slack so I can insert the bolt through the loosen loop. I could force a bit the loop using the bolt as a lever and then the bolt will come in a vertical position by turning it in the nut. Of course the bolt would have to stay above the tensioning block and still press it down so that would be arranged by having a collar up to the bolt's head. I hope i make some sense, english is not my mother tongue (I can make a sketch of that if needed ) ).

 

[SS369]

Hi knothor.

In your current drawing I see three pins/posts. The two small pins I surmise are to keep the spectra on a straight path for a bit. The third it the tie off post/bollard. If you were to tie, let's say a bowline and put it on the bigger post, run the thread around the path of the device, then you could use a tensioning hitch such as the one I had linked to previously. http://www.animatedknots.com/lightermans/
The Lighterman's Hitch allows for you to pull to tension, and while maintaining that tension tie off. You might try additional wraps before the finish.

Not knotting, but you might consider the type of tightening that a stringed musical instrument uses, either primitive or modern.

SS

[knothor]

My real device only has the bolt currently to press over the several loops and thus tension the system. I don't yet have the two small posts, those might get there with the second iteration of the device.

Here is a better close-up: https://www.dropbox.com/s/j6473fvczxal369/20150629_225350.jpg?dl=0

In fact, because of how the strings go in an figure-8 pattern through the plastic, the more natural way to tie it would be to go straight from the end holes to the bolt as the spectra exits the plastic via an arc hole. I will study the Lighterman's Hitch to see how it applies to this situation.

About the musical instruments way, I tried to stay away from guitar tuning keys. Mostly because I am stubborn to find a simpler solution even if the difference in cost is small.


Edit: I studied a bit the Lighterman's Hitch and I have doubts that it would not slip when tensioning the whole loop. I like that it can be untied but I'd rather have security and no slippage.

Edit2: I have another idea: I think I am going to tie a knot around the bolt but under the block. This way when the block is screwed down it will tension these loop ends as well as all the others. I am left with selecting the winner knot. I think a permanent one would be better, or one that can be untied but which locks under tension, if such a knot exists.

[SS369]

How much force or tension do you think will be on the Spectra?

SS

[knothor]

I have no idea yet. It will have to be able to withstand moving up and down anywhere between 1-5kg. The spectra itself is rated at 40Kg but I don't know in Newtons how many it can resist. It should be pretty tight because, if you look at the "elevator platform" sketch I posted, the lower corner should never touch the wall but it should glide up and down in a vertical line with the upper corner.

Bottomline I don't have a clue. I am building this for the first time. I'll do "tighten a bit - test" cycles until I am happy with the results. If the line breaks that's tough luck, will use two lines per loop but that would mean other parts made because the holes are too small for two parallel strings.

[alpineer]

You're better off to have binding posts independent of the tension block IMO and keep the job of each part separate. I don't like to see unnecessary side forces on that tension block. And you'll be introducing an extra degree of freedom in the line where the knot may have to move under the block to maintain equilibrium or suffer offset forces. I'm not outright saying it won't work that way, just that it might cause some unanticipated and less desirable operating condition.   

[knothor]

I agree with you and I actually like the two post suggestion, 1-2 wraps around each post then knot. That will surely be the case for the second pair.

However for this first prototype I only have the bolt as a post unless I do a straight knot (mid-air) between the two ends. And I think I'd go with first option for this one. I only need a knot for this particular situation as well. I might do one wrap of each end around the bolt before knotting. What are you suggesting?

If this one is successful, by next week I'd be able to have the second pair made (with the modified design).

[alpineer]

That looks like an expensive project, knothor. Be mindful of any modifications that would require you to have a piece remade if the mod doesn't work as expected.

[knothor]

I 3D print these parts, so plastic filament+bearings+bolts/nuts+spectra I guess I come up under $15-$20.

From all the knots that have been mentioned in this thread which go well with the knot around the bolt situation? Or maybe another one is more suitable now that we settled on a way to do the knot for this prototype.

Thanks.

[alpineer]

  I had no idea your costs could be so miniscule. But you left out the cost of designing, and the printing, regardless of who does them.

[knothor]

Yes i didn't count the cost of my time for the design nor the 7+h it takes for this to print but well, sometime the best comes when money aren't the only goal .

[knothor]

Hi guys, didn't have time lately to read and test knots... I would be pretty happy if I can get a suggestion for the over the stud permanent knot made of the two ends of rope.

My beef with most knots is that seldom a permanent knot finishes by pulling the standing ends to tighten it. In most cases, eg double fisherman's knot and many more, you tighten the knot by pulling the working cord, which would introduce too much slack in my situation.

Thanks.

[knothor]

I think I am thinking it the wrong way. I might have to get all the spectra out and start by tying a non-slip knot around a stud, then crochet it through the loops, then end with some sort of knot. The way I have it it simply doesn't work. I have tried a surgeon knot but it slipped under tension. A square knot slipped right away. I couldn't tie a good enough fishermans knot  because that requires pulling the working line to lock the knots on one another and my line is in the channels.

Well... learning experience.

[SS369]

Good day knothor.

Multiple wraps around the post(s) will reduce the strain at the actual knot.
I would tie a tensionless type on the fixed one end (attached) and a hitch with wraps<again, on the to be tensioned end as suggested previously.

The wraps are the key to reducing the strain on the locking aspect of the knot. Plus they help to hold the initial tension.

How tight to you suspect the spectra to be necessary? I am wondering if the post(s) will be able to handle the strain.

SS