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The right rope (Read 4470 times)
Castle7
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The right rope
Oct 11th, 2005 at 9:44pm
 
I built an onager and the ropes kept breaking. Did some searching around and it seems like polypropylene is the best. Here is a site with a little more info about it
http://www.crystalcoastcordage.com/polybrai.html

Can anyone tell me if this is the best stuff for torsion powered engines?
And how do I calculate the amount of tourque being used and how much the rope can take?
If it can stand 750 lbs. of weight, can it stand 750 lbs. of torque?
Understand what I mean? Embarrassed Undecided
Thanks ya'll
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Castle7
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Re: The right rope
Reply #1 - Oct 11th, 2005 at 9:59pm
 
Well, considering the rope was about 3 years old, I wouldn't call it overkill.

Do I want a rope with any elasticity?
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Matthias
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Re: The right rope
Reply #2 - Oct 12th, 2005 at 12:46am
 
If you want to store energy in the rope, you need elastic rope. If your frame is too flexible (it needs to be *really* stiff) you'll end up storing energy in there too, and you might not get it back in useable form.

Nylon would be a better choice than Polypro - I imagine that it is recommended more due to easy availability (hardware store "yellow" rope) than outright suitability.

The rope rating won't really have any direct bearing on the power/capacity of the "spring". You use lots of strands, and are loading them completely differently than a tensile test. Keep in mind that the lengthwise tesion in the rope bundle is going to be many times the force on the end of our arms - you are pretty much building a spanish windlass.

You could calculate all this, with a bunch of assumptions thrown in, or you could use the FS approach and Johnny-Eyeball the whole thing. At least half of all engineering jobs end up being J-E jobs (maybe dressed up a bit) so you certainly won't be in bad company.

You broke one set of cables. Lesson: they weren't big enough. I'd make another set and if they were too tough you can always unwind them?

Matthias
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Smudge
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Re: The right rope
Reply #3 - Oct 12th, 2005 at 8:10am
 
It is messy, but you can go to anyone who deer hunts and they would probably give you sinew that you can spin into 2-ply cordage.  It's quite stretchy.
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Castle7
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Re: The right rope
Reply #4 - Oct 12th, 2005 at 10:26am
 
Two ropes were 3 years old and the other two were 5 I think. That might explain something. Lips Sealed

I can get nylon, but just to make sure, is there anything better and within a reasonable price?
I think I'll pass on using sinew right now. I have zero experiance with it and I don't know were to get it in my area. I'll save it for a later project.

Thanks for all the info.

FYI. My frame doesn't seem to have any flex anymore. When I first built it, that was a real problem. My throwing arm is tapered, light, and strong. I'm not sure what wood I am using but it is working for this sized project. I'm using a sling at the end instead of a bucket. Nearly put a hole in a barn with a golf ball at 100ft. The sling released way to late and gave a very low trajectory. When I made the adjustments and began to cock the arm, that's when the ropes broke. I guess this would really be called a torsion trebuchet because I yet to put the uprights on the frame. That will be todays project I think.

To funslinger: An onager is a catapult. And you are correct, it uses twisted cables to store the energy.
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Curious Aardvark
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Re: The right rope
Reply #5 - Oct 12th, 2005 at 11:40am
 
the main thing is not to use too thick a rope.
You get more power with longer lengths of thinner rope (the more windings the more power). Polypropylene (I didn't think) was all that elastic.

The romans used to use sinew and there is no modern equivalent that matches it - but they used to use miles of the stuff. Probably the best thing you could get for a reasonable price would be thin climbing rope. It needs a bit of elasticity and is readily available and generally pretty tough.
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Matthias
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Re: The right rope
Reply #6 - Oct 12th, 2005 at 2:11pm
 
How big are we talking about here? Do you have any pictures of your setup? I'm not sure that climbing rope would be the best choice, isn't it usually built to stretch on a fall and absorb the energy somewhat? When you fall, you don't bounce all the way back to where you started.

If your machine isn't too big, heavy monofilament fishing line might do a good trick. It is relatively cheap, has very good characteristics for your application and you'd have fine control over the torsion bundle.

Matthias
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Castle7
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Re: The right rope
Reply #7 - Oct 12th, 2005 at 5:48pm
 
It's not to big. Here are a couple pics. Once again, sorry for the quality. It's my web-cam.
...
...

As soon as I'm satisfied with this one, I'm gonna build a bigger one.

I put one of the broken ropes on to show where they go on my design.

And these pics are before the uprights.
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Matthias
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Re: The right rope
Reply #8 - Oct 13th, 2005 at 2:23am
 
Heavy monofilament would work great on that (you'd need lots - pretty much a whole roll). I think Aardvark is right about the number of strands as well, for something that scale the more loops the better. If all you can find is polypro, maybe try unlaying a few meters and then use a single strand rather than three-ply - the laid rope won't store energy as efficiently as parallel strands.

Another option that would be a little less period but "probably" almost effective would be old bicycle inner tubes. You'd have to reall y pack the twist in, as they are a bit "soggy" when first stretched but it might worth a try if you have as many laying around as I do!

Matthias

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TLM
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Re: The right rope
Reply #9 - Oct 13th, 2005 at 3:08am
 
On a BBC  program where they built a copy of a roman  "what was it called" they ended up using polyester rope as it is apparently fairly close in elastic properties to sinew.

TLM
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Smudge
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Re: The right rope
Reply #10 - Oct 13th, 2005 at 8:01am
 
The onager was put under more stress than the trebuchet, so when it threw its projectile it literaly kicked like mule in that the back end would pick upmin the air with each shot.  Since it kicked like this it could not be pinpointed at and exact location on a castle wall, making its effectiveness questionable.  The trebuchet could also fling larger stones.
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Matthias
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Re: The right rope
Reply #11 - Oct 13th, 2005 at 2:09pm
 
I think in use you would pre-tension the spring and then winch the arm back for each shot, so no handicap there. It is also a small, compact and easily moved piece of equipment, while a treb would have to be pretty much assembled on site. More like field artillery?

Matthias
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Castle7
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Re: The right rope
Reply #12 - Oct 15th, 2005 at 9:56pm
 
Only got a sec, but wanted to let you guys know I am building a new onager. It's a little bigger. It's weighs about 45lbs. so far. I only have a bare frame built now. Hopefully I will get some rope tommorow.


OVERKILL BABY
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Re: The right rope
Reply #13 - Oct 16th, 2005 at 10:29pm
 
Go get some nylon or polypropeline. Rope does not need extreme "elasticity" to store energy. You are thinking of a rubber band. It is far better for a rope to have a little stretch under a lot of strain, and resist the stretching.  Polypropylene has around 8percent of stretch, after all the set it can take. Sinew has somewhere around 5percent. Nylon is very similar to polypropelene. Get some nylon. It works just fine. When you thread the seige weapon, pull it as tight as you can. This will reduce the amount of twisting you have to do and therefore ought to reduce the chance of the rope shearing on itself. Make sure the frame has no friction/wear spots. If there is anything sharp in the framework, but you can t  get rid of it, wear eye protection.

Another thing. Use "selection" wood. Use ash or any other light strong wood for the arms. Use oak or any dense strong wood for the trigger. Use elm or oak for the frame. Ash should work to, but you seem to like overkill. Also, do some more research on seige weapons and such.
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Castle7
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Re: The right rope
Reply #14 - Oct 17th, 2005 at 12:00pm
 
Well, at this point I'm just using what ever I can get. I've done a fair amount of research and will continue to do more. Do you believe there is anything I should look for in particular?

I've purchesed some nylon rope. There are no sharp edges or points and friction points, I believe, are minimal. I am having a little trouble with the skein so far. Can't seem to find proper parts. As you recommended I will do more research on the skein and the trigger as well. Mine is very simple. It works well and is safe but will be hard to pull under a heavy load.

Thanks for all your help, everybody
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