advice on knots and ropes

[tommaso]

Dear experts,

I wished to create a tool for the gym which has the structure as shown in the attached picture.

Essentially a tube suspended to a swivel (there will be other ropes connected to the tube trough pulleys, which will have to support the weight of a moving athlete).

I'd really like to see your advice and suggestions on how to connect (green lines) the swivel to the eyelets attached to the tube. Like for instance what kind of rope, what kind of knots to be used. How to pass the rope around the various rings. (If possible and advisable, I'd like to have a double pass on each side.)

Please, feel free to give your advice and suggestions on how to create this object (and not looking a complete incompetent).

Thank you!

[roo]

Dear experts,

I wished to create a tool for the gym which has the structure as shown in the attached picture.

Essentially a tube suspended to a swivel (there will be other ropes connected to the tube trough pulleys, which will have to support the weight of a moving athlete).

I'd really like to see your advice and suggestions on how to connect (green lines) the swivel to the eyelets attached to the tube. Like for instance what kind of rope, what kind of knots to be used. How to pass the rope around the various rings. (If possible and advisable, I'd like to have a double pass on each side.)

Please, feel free to give your advice and suggestions on how to create this object (and not looking a complete incompetent).

Thank you!

We've had a similar thread before:

http://igkt.net/sm/index.php?topic=1858.0

I'll note that your angles are quite severe (if to scale) and will serve to multiply the load on the angled line.  If this is supporting someone, it'd be good to get someone with the proper training (e.g. an engineer) look at the details of your system in person and run some numbers. 

[tommaso]

Thank you Roo,

Actually the angles are not in precise scale, as I did not want to make the picture too tall.

I guess there is a good degree of freedom in choosing those angles, and this would be clearer seeing the tool in action.

Here is a video where a similar tool is shown: https://www.youtube.com/watch?v=98sr7TO7t94
(Security is not here an overwhelming concern, as the tool is not used for climbing, but very close to the floor and over soft gym mats. I just wish the tool to be done with some reasonable common sense).

I have been doing some research by myself on YouTube, and reading about ropes strength.

I am under the impression that a polyester rope of 1 cm diameter could probably work fine (I have seen they can hold more than 500Kg).

As to the knots to use I am not clear at all. I have seen a few videos on knots and it seems the the so called figure 8 and the bowline are quite popular. However I'd like, for greater security, if possible to run and tie the rope double. So that the swivel and each eyelet are connected by 2 pieces of rope. See the picture attached to see what I mean. I am not sure how to tie the rope to the eyelet and close the loop itself.

Clearly, I am open to better ideas to join the parts. Thanks a lot for any ideas.

[roo]

Thank you Roo,

Actually the angles are not in precise scale, as I did not want to make the picture too tall.

I guess there is a good degree of freedom in choosing those angles, and this would be clearer seeing the tool in action.

Here is a video where a similar tool is shown: https://www.youtube.com/watch?v=98sr7TO7t94
(Security is not here an overwhelming concern, as the tool is not used for climbing, but very close to the floor and over soft gym mats. I just wish the tool to be done with some reasonable common sense).

I have been doing some research by myself on YouTube, and reading about ropes strength.

I am under the impression that a polyester rope of 1 cm diameter could probably work fine (I have seen they can hold more than 500Kg).

As to the knots to use I am not clear at all. I have seen a few videos on knots and seems the the so called figure 8 and the bowline are quite popular. However I'd like, for greater security, if possible to run and tie the rope double. So that the swivel and each eyelet are connected by 2 pieces of rope. See the picture attached to see what I mean. I am not sure how to tie the rope to the eyelet and close the loop itself.

Clearly, I am open to better ideas to join the parts. Thanks a lot for any ideas.

Doubling up the line won't increase security, but it will add strength.  It might be good to use a single line with an appropriate load rating and a secure hitch at each terminal, so that the rope surface doesn't jostle around against the terminals and become frayed. 

Here's a single pass-hitch with excellent security:  http://notableknotindex.webs.com/gnathitch.html

[tommaso]

Thanks a lot Roo.

I had never heard about that knot, and I am very happy to learn about it! It seems also quite simple.

I have tried to summarize your suggestion in the attached picture. Let me know in case I got it wrong (or if you have additional suggestions).
 
Thank you very much!

[Sweeney]

The video referred to shows a line angle of less than 45 degrees - or possibly 45 degrees at most. I would go with your own suggestion and tie a rope as a sling between each pair of attachment points using double fisherman's knots to join the ends. As Roo rightly says this does not add security but adds significantly to the strength. Personally I would use 10.5mm climbing rope to take this sort of punishment - dynamic rope has elasticity which may be helpful - if not static rope has almost none. This video might be of interest https://www.youtube.com/watch?v=4ESdgrCJrek

Sweeney

[tommaso]

The video referred to shows a line angle of less than 45 degrees - or possibly 45 degrees at most. I would go with your own suggestion and tie a rope as a sling between each pair of attachment points using double fisherman's knots to join the ends. As Roo rightly says this does not add security but adds significantly to the strength. Personally I would use 10.5mm climbing rope to take this sort of punishment - dynamic rope has elasticity which may be helpful - if not static rope has almost none. This video might be of interest https://www.youtube.com/watch?v=4ESdgrCJrek

Sweeney

Thank you very much Sweeney for your suggestions.

I hope you don't mind if a make a few questions to understand better all what you are saying.

You are right noticing that in this kind of tool the angle is probably not going to exceed 45. I'd say possibly around 30 or even less. I am not sure "why", but an angle bigger than 45 would just not "look" right for this kind of application.

I find very useful your distinction between:

 - climbing (=dynamic?) rope
 - static rope

in fact I had been looking at sell advertisements for various types of ropes, but I had not actually realized that the
"climbing" ropes have some elasticity which non climbing rope do not have (I hope I got this right).

[I have also seen ropes denominated "bungee ropes" (but I guess that those are yet another type with much greater elasticity) which probably are not suitable here.]

I understand the double fisherman's knot (in picture) is quite effective in "welding" the rope (and maybe the 2 knots on the rope wear less easily than close to the metal ?).

I have however a concern with the eyelet. In addition to the double fisherman's knot to close the loop, I am under the impression I would need some additional knot around the eyelet to avoid that the rope could allow the eyelet to slide up and down within the loop when the person attached to it moves his legs (for instance to perform the so called "flares", eg: https://www.youtube.com/watch?v=AqnvDQSWwwc).

If this is a concern, how could I also fix the eyelet at precise distance within this loop ?

[Sweeney]

First I should explain the difference between the 3 ropes in a bit more detail viz:

dynamic rope - used for climbing where a fall would result in serious injury if the rope were inelastic ie the climber needs the rope to absorb some of the shock of the the fall (see bungee rope below which is a more extreme case!).

static rope - also used in climbing but as an abseil or descent rope where minimal elongation is required.

bungee rope - made from bound rubber strips so that somebody jumping off a bridge say has maximum elasticity in the rope and bounces several times at the end of the deliberate fall. I have also seen this used a mooring rope (river cruisers in Chester on the river Dee) - allows the boat to be brought to a gentle stop more easily. I would imagine no use for your application.

Ropes sold for general use will have a limited elasticity depending on the material used and method of construction (nylon stretches more than polyester for example) but climbing ropes will be certified by the manufacturer - both strength and elongation percentage.

Looking at the configuration the 2 ropes are attached separately to the eyelet on the bar so if you use a sling of rope between eye and swivel it would be easy enough to add a clove hitch at each attachment point before tying the double fisherman's to prevent movement - if indeed movement along the sling does occur?. This assumes that the eyelet is big enough to allow 2 thickness of rope.

Rope wear is an issue whatever you do and regular inspection is essential. It depends on how much movement there is between rope and metal when the rope is under tension - a clove hitch puts 2 wraps around the eyelet and swivel and is intended to have tension on both ends so will work within a sling. This will stop some movement but it is unlikely to make much difference to the life of the rope; on the plus side you appear to be using about 4 metres at most so cost may not be a primary consideration?  You don't necessarily need climbing rope as such but the combination of shallow angle and repeated loading means that you will need rope to stand up to considerable stress.

Hope this helps

Sweeney

[tommaso]

Thank you very much Sweeney and Roo!

Your input has been very helpful and I have actually learnt much more than I expected about knots and ropes.

I am still waiting for some material I have ordered (bar and various straps and bolts/nuts) and when it all arrives I will be trying the various configurations you have suggested.

In summary we have:

- A simpler solution with one line tied to the swivel and the eyelet (this for each side)
using a gnat knot  (I expect this node to become popular in future, given its simplicity and such simple name!)

- A (possibly stronger solution, but more complex to make) with one sling of rope (for each side)
closed with a double fisherman's knot, possibly after fixing it at the eyelet with a clove hitch knot.

(I hope I got all right)

[roo]

Thanks a lot Roo.

I had never heard about that knot, and I am very happy to learn about it! It seems also quite simple.

I have tried to summarize your suggestion in the attached picture. Let me know in case I got it wrong (or if you have additional suggestions).
 
Thank you very much!

That diagram looks good.  Considering the wild motion this will see, I think that approach will work nicely.

[agent_smith]

I'd really like to see your advice and suggestions on how to connect (green lines) the swivel to the eyelets attached to the tube. Like for instance what kind of rope, what kind of knots to be used. How to pass the rope around the various rings. (If possible and advisable, I'd like to have a double pass on each side.)

Please, feel free to give your advice and suggestions on how to create this object (and not looking a complete incompetent).

What solution did you end up using? (photos??)
Is there a presumption is that the system must be capable of supporting a person up to 100kg? (or more?).
What are the consequences of knot failure - how far could the gymnast fall? Is there risk of of serious injury/permanent disablement in the event of knot failure resulting in a fall?

If the answer is 'yes' - then I do not recommend the use of gnat hitches in human life support applications.

Mark G

[tommaso]

Hi Mark,

thank you for your inquiry. I have been waiting for the various parts and I have put together a very preliminary prototype,
just to have a general idea of how it works and the measures of the bar and ropes and other parts.

The most difficult task was to make the holes in the steel tube. That is an ordinary spare tube of a vacuum cleaner, which I hope could be of more usefulness because it is telescopic. However, in my first preliminary test I discovered the telescopic mechanism is not able to hold in place with the weight of a person and therefore I have inserted a bolt, making it of fixed size.

Another thing was attaching the pulley to the tube. The pulleys that were available seemed to allow on for "lateral" fixing, so I had to mount them laterally and use the bolt itself as the hook where to attach the rope.

For now, I have simply attached a rope above (an (ugly) overhand knot above, and 2 gnat hitch knots below). There would not be room for a double pass of the rope, as the pulley is not large enough. This is just temporary, I would like your advice of course. The white rope is 1 cm wide. The blue rope is 6 mm (a very tough one).

Similarly, 2 gnat hitches are holding the ankle straps.

Everything is just temporary and prototypical (just hanging it in my garden), so any idea for improvement is very welcome.

In the meantime, I have been reading/learning more on knots and also learned a few new ones. Find them very interesting and it's kind
of shameful not having learned this stuff way earlier (I think they should teach them in primary school).


>Is there a presumption is that the system must be capable of supporting a person up to 100kg? (or more?).

I would say so. Even if the legs of a person weigh way less than that, of course one would want a reliable training tool (it can move pretty fast and therefore the energy involved can be large.).

>What are the consequences of knot failure - how far could the gymnast fall? Is there risk of of serious injury/permanent disablement in the event of knot failure resulting in a fall?

Well gymnasts take some (controlled) risks all the time and therefore usually one is used to take appropriate precautions (like good mats and so on). Anyway, nothing can be completely excluded, even though I think that very serious injuries should be quite unlikely.

Anyway a knot failure while training would certainly not be a pleasant experience, even working on soft mats.

 

[roo]

If the answer is 'yes' - then I do not recommend the use of gnat hitches in human life support applications.

Mark G

On the last page of your Bowline Analysis paper, you recommend four loops for life support.  So I picked the simplest one*, which is the Lee Zep Bowline.

I tied and firmly set the Lee Zep Bowline to one end of a carabiner, and a Gnat Hitch on the other end.   Both were tied in Bluewater II rope with a short 3" tail.  Keeping the carabiner in the middle, and my hands an equal distance from each knot, I flogged the rope against the ground until the Lee Zep Bowline came undone after 22 impacts.  The Gnat Hitch remained firmly tied.

I repeated the test with a traditional Figure 8 Loop against a Gnat Hitch.  The Figure 8 Loop came undone after 52 impacts.  The Gnat Hitch remained firmly tied.


*I excluded the Scott's Locked bowline due to shape stability/capsizing problems that could occur with the average tyer depending on dressing, as has been discussed before:

http://igkt.net/sm/index.php?topic=4480.msg32318#msg32318
http://igkt.net/sm/index.php?topic=19.msg22509#msg22509
UPDATE:  Loop snagging vulnerability found: https://igkt.net/sm/index.php?topic=6773.msg44594#msg44594

update:  The same test repeated with your EBSB Bowline against a Gnat Hitch (all conditions the same):  The EBSB came undone after 66 impacts/shakes.  The Gnat Hitch remained firmly tied.

[agent_smith]

Roo - your homebrew tests are largely meaningless.
I have read the back n forth posts between you and others in this forum in the links you provided. Most of your testing has conditions attached - that is, in order for another tester in a different part of the world to try to reproduce your own results would be difficult to near impossible.

Further to this, can you explain how flogging a test article against the ground has any real-world application? And random flogging might just favor a particular structure due to impact and crushing against a hard surface - giving rise to what I refer to as a 'false positive'.

The Lee Zep Bowline works remarkably well as part of a dual clip-in system for indoor climbing gyms (in lieu of a Butterfly - which jams after a period of time in a climbing gym environment). You also might have missed the key fact that the Lee Zep Bowline is captive within the rope system - no end is free - the tail continues and then forms another connection. Have another closer look at my paper to see this crucial fact that you overlooked.

The use of a gnat hitch or any unsecured hitch for mission critical human fall-arrest purposes is not something that you should be promoting to the world in an open forum such as this.
Perhaps one good example is the rope access industry - who routinely use 'cows tail' lanyards fitted to their full body harnesses. ABoK #409 (Poachers knot) is the universally preferred method to secure an end of a rope to an object (eg a carabiner) in the mission critical rope access systems. You'll find some rope access operators referring to this structure as a 'scaffold knot'  (which actually is #1120 triple overhand noose rather than double overhand noose #409 version).
Link: http://www.animatedknots.com/poachers/index.php
Link: http://itrsonline.org/wordpress/wp-content/uploads/2015/11/Luscinski-Knotted-vs-Sewn-Terminations.pdf

If you turned up to an IRATA or ARAA/SPRAT rope access assessment with your lanyards terminated with a gnat hitch instead of a secure noose (#409 or #1120) - you will fail.

I have zero interest in debating this matter with you personally any further.

Mark G

This lack of interest in engaging with you is also in respect of the OP stated desire to find a solution to his issue.

Edited: Links should be working now...

[roo]

Roo - your homebrew tests are largely meaningless.
I have read the back n forth posts between you and others in this forum in the links you provided. Most of your testing has conditions attached - that is, in order for another tester in a different part of the world to try to reproduce your own results would be difficult to near impossible.

Further to this, can you explain how flogging a test article against the ground has any real-world application? And random flogging might just favor a particular structure due to impact and crushing against a hard surface - giving rise to what I refer to as a 'false positive'.

The Lee Zep Bowline works remarkably well as part of a dual clip-in system for indoor climbing gyms (in lieu of a Butterfly - which jams after a period of time in a climbing gym environment). You also might have missed the key fact that the Lee Zep Bowline is captive within the rope system - no end is free - the tail continues and then forms another connection. Have another closer look at my paper to see this crucial fact that you overlooked.

The use of a gnat hitch or any unsecured hitch for mission critical human fall-arrest purposes is not something that you should be promoting to the world in an open forum such as this.
Perhaps one good example is the rope access industry - who routinely use 'cows tail' lanyards fitted to their full body harnesses. ABoK #409 (Poachers knot) is the universally preferred method to secure an end of a rope to an object (eg a carabiner) in the mission critical rope access systems. You'll find some rope access operators referring to this structure as a 'scaffold knot'  (which actually is #1120 triple overhand noose rather than double overhand noose #409 version).
Link: http://www.animatedknots.com/poachers/index.php
Link: http://itrsonline.org/wordpress/wp-content/uploads/2015/11/Luscinski-Knotted-vs-Sewn-Terminations.pdf

If you turned up to an IRATA or ARAA/SPRAT rope access assessment with your lanyards terminated with a gnat hitch instead of a secure noose (#409 or #1120) - you will fail.

I have zero interest in debating this matter with you personally any further.

Mark G

This lack of interest in engaging with you is also in respect of the OP stated desire to find a solution to his issue.

Edited: Links should be working now...

Wow.

Anyway, since you brought up the #409 noose, I again repeated the test with Bluewater II, and the firmly-set #409 (with a slightly longer tail than 3") came undone after 44 shakes/impacts.  The Gnat Hitch (with a 3" tail) on the other side of the carabiner remained firmly tied.  If you'd like more specifics on the test (amplitude of shake, etc) feel free to PM me so you can reproduce outcomes.

I don't mean to divert this thread, but I hope you can give different approaches a fair shake (pun intended)