Working against gravity ...

[Bronchen-Braten]

Hello! I am looking for the simplest and most practical solution for a (probably very common) problem in terms of gravitation.
Look at the following situation:


(click for enlarged version)

The rope (yellow, idealized) is attached to the beam at two points and is joint in the middle above. Below the beam there is an additional weight attached. The green arrow shows the direction in which gravity pulls the weight, the red arrows show what I want to avoid - the rope sliding off into the middle. Hence the orange rope is there to pull it in the opposite direction. Now in reality this would all bel very wobbly and the orange rope could easily slip to the top or the bottom, as there is nothing there to hold it in place (the whole problem wouldn't exist if there were either holes in each end of the beam or if there were rings attached)

Now I'm looking for a good, functional way to tie the rope around the beam without the possibility of it slipping. Of course it couldn't be tied the way it's illustrated in the picture above by the yellow line, singe the rope actually splits up there when it goes around the beam. That's what needs to be done with a knot (or multiple ones), plus there should be an equivalent for the role of the orange lines - It's a single piece of rope and both ends come together at the bottom, where the weight is attached. Does anyone have an idea?


P.S. If you're wondering why I have to have this specific kind of construction - I built a swing (an experimental kind) last year that could turn in various directions. It was fun to use (with safety belts) but I took it apart now as it was a bit too dogey, but I want to build it again, safer. One feature of it was that it was be able to rock back and forth vertically. It's a bit hard to explain but it's not the essence of the problem. See this photo: http://img339.imageshack.us/img339/8768/img3883d.jpg
As you can see, the knot-construction I made there worked, but it wasn't very professional to say the least.

[jcsampson]

Edit: After examining this idea using a piece of rope, I have realized that Three-Coil-Ring Fixed-Gripper Coil Hitches would be significantly better (i.e., easier) than Four-Coil-Ring Fixed-Gripper Coil Hitches (for the method that uses the Two-Coil-Ring Fixed-Gripper Coil Binders), so the appropriate edits have been made below. Also, I have added a new knot diagram (and its associating edit, to the referenced thread) that shows a second method of locking the Fixed-Gripper Knot, which is a much better way to lock, because it is rock solid yet easy to untie.

The first thought that comes to mind is that you should drill holes near the ends of the wood and hitch the ropes through the holes.

However, if you don't want to, or can't, do that for some reason, then one possible easy solution is to try a Four-Coil-Ring Fixed-Gripper Coil Hitch on all ropes (top ropes and separate bottom ones) attached to those areas of the wood. If the amount of pull towards the center is so great that these hitches wind up showing some slide, then you could try adding one or more Four-Coil-Ring Fixed-Gripper Coil Binders ADJACENT to those Coil Hitches, to the sides near the center of the wood, to help keep the hitches from sliding inwards.

If this leaves something to be desired, then you could try the following, which would take more work, but give you exactly what you have in your diagram:

- Using small, separate pieces of rope, tie a Two-Coil-Ring Fixed-Gripper Coil Binder around the wood, near each of the two ends of the wood. In this particular application, which has unusual safety demands, I would suggest locking the Fixed-Gripper Knots (using the "Fixed-Gripper-Locked Fixed-Gripper Knot") (See also the alternative to locking at some point below.)

- Using small, separate pieces of rope, tie an end catch around each of the two ends of the wood by tying the pieces to the Two-Coil-Ring Fixed-Gripper Coil Binders, using Three-Coil-Ring Fixed-Gripper Coil Hitches

- Finally, using Three-Coil-Ring Fixed-Gripper Coil Hitches, attach the upper suspending ropes and the separate lower suspending ropes to the Two-Coil-Ring Fixed-Gripper Coil Binders, at the appropriate spots shown in your diagram

Of course, the pull of gravity will attempt to pull the Two-Coil-Ring Fixed-Gripper Coil Binders towards the center, so they might take on (or try to take on) sideways "V" shapes.

As an alternative to locking the Two-Coil-Ring Fixed-Gripper Coil Binders, you could make them using the Fixed-Gripper Slide-and-Grip Hitch Variation, instead of in the usual way. This should hold against a good amount of force (as long as it's in the context of a "Coil Binder") and will remain perpetually adjustable.

Of course, always tie and test at your own risk. . . .

See

http://igkt.net/sm/index.php?topic=1839.msg12439#msg12439

for info about the Fixed-Gripper Constructs.

JCS

[roo]

Hello! I am looking for the simplest and most practical solution

Drill holes in the beam.  Protect the rope with rounded-lipped bushings if needed.  Why are you avoiding the blindingly obvious solution?

Also, make sure your medical insurance is paid up.  

[Bronchen-Braten]

Thanks for the replies!

I guess you're right, I should have said I was looking for the most awkward solution that won't get someone's neck broken Yes, drilling holes into it would solve the problem, but I'm afraid the beam-thing would loose its stability. It's not a wooden plank, it's made from metal, butit's hollow, so I'm not sure about it.

I'll definitely look into the solutions you suggested, jcsampson. Might take me a while to wrap my mind around though, but that's fine. I may reply again if I can't manage. I first replaced the upper rope with a new one today.

A safety-inspector wouldn't give green light on the whole construction anyway, that's for sure ... But I figured at least the knots could be done professionally. I did actually manage to do a couple of 360? vertical flips with the old one. I put the "cage" around it so you couldn't fall on your head, should the gondola decide to fly off to a field trip at some point after all ... but that's about as far as predictability goes. It's gotta be enter at your own risk I guess - note the sign in the picture (up in the tree)

[roo]

Yes, drilling holes into it would solve the problem, but I'm afraid the beam-thing would lose its stability. It's not a wooden plank, it's made from metal, but it's hollow, so I'm not sure about it.

A standard schedule 40 thickness pipe or a piece of wood that size can be had cheaply, or maybe even free if you look around the net.  A figure 8 stopper knot*  below the pipe or beam on each side would keep the pipe or beam from sliding down the two ropes, as long as the holes are sized correctly.  

The pipe or beam would just be holding the two ropes apart, which isn't very demanding in view of the weight.  The most critical aspect would be protecting the rope from sharp or tight-radius corners, as I mentioned before.

Have fun.

*A Butterfly Loop or a Span Loop can be used as an on-the bight stopper, if desired:
http://notableknotindex.webs.com/spanloop.html
http://notableknotindex.webs.com/butterflyloop.html

[Bob Thrun]

I can't follow your diagram.  I simply can't see yellow lines on a white background.

[jcsampson]

Hey Bronchen-Braten,

Using some rope, I've looked into this application with some greater detail and have made a few edits to my original post in this thread.

Some additional thoughts: The Coil Binders may need to be adjusted slightly loose around the wood, so that you will be able to position and tighten the Coil Hitches adequately. . . .

JCS

[DerekSmith]

Hi Bronchen-Braten,

Welcome to the forum and thank you for bringing an interesting problem to us.  It has a number of parts that cover important issues with rope usage.

First up is a Don't - don't run rope over hard metal corners or around square box section - for two reasons.  The first is that it is virtually impossible for the rope to grip the flat sides of the square section, and the second is that all the grip and pressure is concentrated on the corners which can create such huge pressure points that it can lead to rope failure.

Second up - learn about vector forces and 'rope leverage' - That triangle formed by the rope between the cross beam and the top twist shackle is a potential weak point because of rope leverage.  For example, if the cross beam is seven foot long and the twist shackle is fixed with rope so that the beam hangs just two feet below it, then the rope leverage causes whatever weight you hang on the swing to be doubled in that top triangle of rope - 200lb load - 400lb tension in that top triangle of rope.  To keep this leverage to a sensible level, don't make the distance from shackle to beam too short - never less than half the length of the beam.

Third up - use turns instead of knots.  Turns grip, while knots act as weakening points.

Finally KISS - or at least keep it as simple as possible (KISAP).  A well designed wrap will be stronger and grip better than a great feast of knots and holes.

So concluding - Swap the metal bar for a round wooden pole - Fix the rope to the shackle with round turns or a Cow hitch - Fix the rope to the pole using a half hitch (or at very most a Constrictor or a Strangle if you are worried about unloaded slippage) - Keep the pole to shackle distance at least half the length of the pole, more if possible - Then finally use round turns to start your fixing to the 'gondola' and only use 'knots' to tie in the ends.

Hang cool.

Derek

[roo]

Fix the rope to the pole using a half hitch (or at very most a Constrictor or a Strangle if you are worried about unloaded slippage) -

I think you are overestimating the ability of a half hitch, Constrictor Knot, and Strangle Knot to resist lengthwise force.

[DerekSmith]

Sorry Roo, no overestimate, I tied the bindings and tested them before posting the suggestion.

I tied it using 3mm polyprop braid using a 5ft long, 1" diameter timber dowel.  I tied it with 30 degree and 45 degree top rope angles, and hung 100kg on it with no slipping towards the centre of the pole of any of the proposed bindings under load.

I learned from the master of grip about the power of round turns - from your good self in fact.

However, it is well documented that this type of force system is heavily dependent upon there being a large difference between the diameter of the cord and the diameter of the pole.  When I tried this setup with 8mm poly laid rope on a 1" dowel, it refused to hold under even modest loads, but 6mm rope on a 3" pole held firmly.

There are two reasons why this simplest of bindings works.  The first is that these bindings are self gripping - the greater the load the more they grip.  The second features an element key to the KC Hitch.  The two loaded lines enter and leave the binding on the same face of the pole, the tension forces meanwhile have been transferred around the pole via the two wraps.  Due to the tension in the wraps, there is significant friction between the turns and the pole.  The force inwards attempting to make the binding slide towards the middle of the pole acts on one side of the binding and pulls that side slightly towards the middle.  In doing so, it stretches the cord and so increases the tension in it, further increasing the grip in the turns at the back of the pole.  As there is no direct force pushing the back wraps in towards the middle, the frictional grip easily stays in place and acts as an anchor for the rest of the binding.

This angling and stretching of the turns also gives a clue as to the reason why large diameter rope on small diameter spars does not grip, because the large diameter rope easily transmits force around the spar to the back turns and so can 'pull' them down the spar without first stretching the rope.

Derek

[roo]

Sorry Roo, no overestimate, I tied the bindings and tested them before posting the suggestion.

Excuse me if I missed it in your reply, but what I especially worry about is the effect of both ends of the Constrictor (for example) being loaded inward at an angle to produce a lengthwise force along the beam.

[jcsampson]

"Fix the rope to the pole using a half hitch (or at very most a Constrictor or a Strangle if you are worried about unloaded slippage) . . ."

To verify that I am interpreting your reply correctly, do you mean to have, on each side of the swing, one piece of rope going from the top hardware to the swing's basket and--on the way--have it wrap around the pole in Half-Hitch fashion?

Are you recommending also that Round Turns be used in tandem with this Half Hitch, to increase the rope's grip on the pole?

JCS

[Dan_Lehman]

The beam part of this contraption that is diagramed (and maybe
also pictured in that photo) is shown as having a pronounced
rectangular cross-section -- is this indeed the case?

If the photo'd beam is the one of immediate concern, might it
be possible to orient the cross-section'd aspect to be oriented
with the wide sides parallel to the ground / horizontal ?!  If so,
then a simple solution in both tying and material --in that it
uses just a single piece of rope, and can be knotted in without
ends-- is to implement those end-of-beam connections very
nearly matching the diagram with Slip-Knots:  insert the beam
into the Overhand knot to push into the slip-bight.  This can
knotting can be *beefed-up* with an extra turn cast into the
Overhand-knot part (a full turn of the belly).

Now, devoting individual pieces of cordage to particular tasks
(e.g., capping the ends, support to the top, ... to below) will
give greater control and easy adjustment.

--dl*
====

[DerekSmith]

Roo and JCS

Here is an image of the Clove on one end of the pole.  It was tied with a 30 degree angle for the top rope to the shackle, but when loaded the cord stretched and the angle photographed is nearer 35 / 40 degrees.  The hitch is under heavy load on a fairly shiny timber pole.



Under load you can see the loaded ends have pulled in towards the centre of the pole, but the backs of the wraps stayed put.  The angled line of the wraps increases their length, so puts them under greater tension and therefore creates even greater grip.



I tied the other end with a constrictor, although a strangle would probably have been better.  This shot also shows that there is a significantly greater force in the top cord than in the bottom cord because of the leverage caused by the very sharp angle of the top cord.



Derek

[roo]

Roo and JCS

Here is an image of the Clove on one end of the pole.  It was tied with a 30 degree angle for the top rope to the shackle, but when loaded the cord stretched and the angle photographed is nearer 35 / 40 degrees.  The hitch is under heavy load on a fairly shiny timber pole.

Did you notice any movement as load was cyclically applied and released for a while?