Rope theory question: For 3 strand rope, is the count of individual strand twists the same as the count of turns laying up the strands?

[Tim Holt]

Hopefully the subject is clear.

 

I'm currently building a ropewalk using some motors to drive both the initial twists as well as subsequent laying up.  The motors have optical encoders, meaning I can know rotation counts - exactly how many twists I put on.

 

It's not complete yet so I can't test this, but on a Syren Rope Rocket style setup, in theory is the count of twists for step 1 (individual strand twists) the same as the twist count of step 2 (laying up strands)?

[Chuck]

No i doubt that at all.  And the counts on both ends will vary significantly depending on the type and thickness of the rope.  It will also vary depending on how many strands per hook there are and based on the amount of time twisting on each side it always seems as if there are more when you twist the strands together.

[Gregory]

The laying up is only a small fraction of the twisting of the strands, and it also depends on the length of the rope.

The longer the rope, the more twists in the lay up.

 

I have a more traditional rope walk with motor driven whorls and a looper that is driven by tension. I would guess the yarns/threads are twisted hundreds of times for each twist in the strands in the layed up rope.

[Tim Holt]

7 hours ago, Chuck said:

No i doubt that at all

 

OK interesting.  I was watching your demo videos, and noticed your initial twist time with the drill is pretty short (about 9 seconds) but then it's geared up so a lot more revolutions occurring.  Then the laying up is much longer (20+ seconds), but then that's just 1:1 with the drill revolutions.  That's not very precise given the speed variation of the drill, but it got me curious just how many actual revolutions were occurring in that video example - or rather what the ratio was.

 

I'm just wondering if there isn't some base theoretical ratio for different setups for rope size, thread type, etc.   Since I can count my revolutions precisely, it would be nice to dial in a desired count for each step based on that ratio plus how long of a setup I was using.  Sort of like having a torque wrench and a bolt torque table.

Edited July 27, 2021 by Tim Holt

[Chuck]

I dont think you can get that scientific about it.  Especially because it wont matter much in the end.  This is definitely a case of "over-thinking" it.  I am sure the number of actual turns will vary and still produce excellent rope.  Its akin to  wanting to know how many grains of sand is in the sheet of sandpaper you are using to sand your hull and if it takes the same number of grains at 220 grit vs 120 grit to smooth out the rough spots, but only smaller in size.    Its neat trivia, but its not something you need to know to produce a good quality rope.   It more important that you can feel when its laid up right.....and see when its laid up right....

 

Just my two cents after making thousands of miles worth of rope over the years.  If you get bogged down with the math its gonna get weird.   Just give it a go and see what the end results are.  I can say this only because I am literally making rope today and with your question fresh in my mind,  I had to chuckle as I was doing so.   

 

I make 20-22 foot lengths and the drill and ropewalk turns thousands of time over that minute or two.   The Drivers I use are rated at 3500 rpms.  Was it 10,020 revolutions or possibly 10,090....It isnt really going to matter.   Having Math and Science and engineering kids in the family they laugh at me when I tell them I cant even imagine thinking with that side of the brain.   The side of the brain I use just starts to hurt.   But maybe the more analytical minds can help you find out.  I dont even want to imagine that mathematical equation.  It would probably have more letters than numbers...LOL.

 

I started running through the factors in my head.   

 

It depends on how tight you tie off your thread on each hook.  If its not the same each time then that throws off the math.  If you tie off the thread just a centimeter shorter for the next rope....it throws off the equation.  Then there is the thickness of thread.   How many strands?   If you lay up cable with multiple strands and change the direction of the drill?   My head hurts.

 

I will go with feel and sight every time.   It takes about 8 minutes to lay up a 20ft length.   If it doesnt come out right I will toss it and just make another and adjust for humidity and wind direction.

 

An interesting question and take on rope making none the less.

[Gregory]

50 minutes ago, Tim Holt said:

it would be nice to dial in a desired count for each step based on that ratio plus how long of a setup I was using. 

Like Chuck says, you can over think it, and gain absolutely nothing..

 

FWIW, with my set-up, when you have twisted the yarns enough, the rope starts to lay up..

This will vary with the type of thread..  Counting would be pointless..

 

I have been curious about what the RPM of my whorl gears are..  I might get around to checking it someday..

 

 

 

Edited July 27, 2021 by Gregory

[Chuck]

In all seriousness however,  it is my opinion that a far more reliable method for telling how many turns is adequate when working the headstock (turning the individual strands) is the distance the headstock travels plus the feel in the thread after twisting it.

 

For example, with the thread I am using as I write this.  I am laying up some tiny rope....012 rope.

 

Three stands.   I start with the head stock and tail stock 26 feet apart.   When I twist the three strands initially the head stock walks or travels three feet.   At that point I feel the tension in the thread.  Because I have done this more than a couple of times, I can feel when the appropriate amount of tension is built up in the threads.

 

Then I move to the tailstock and turn the strands together.  Same thing really.   I gun my driver at 3500 RPMs until the tailstock backs away as the rope lays up.  Then it starts returning to the original position.  When I reach that point I stop and feel the tension in the rope.   If it feels right and the lay of the rope looks right.   I know its time to stop and the rope is completed.  

 

If after I cut the rope free and it unwinds a bit and the lay looks too loose....easy fix.  I just increase the initial revolutions of the headstock on my next 8 minute rope creation.

 

For me the feel of the line is most important after experimenting to find what the correct "walking length" for the head stock is.   And the results are so surprisingly consistent once you repeat them.

 

This is completely different when I make a different size of rope with a different size thread.  I dont think the math would ever be the same for every size and length.   Maybe it is though.  I just cant believe that it would be.

 

 

 

 

[Gregory]

Like you, I don't want to struggle with these numbers, but you might find this interesting..

 

With your Rope Rocket at 26 feet  moving 3 feet with tensioning, that comes out to about 11.5%.

 

I checked my rope walk, and with 55 inches of thread, it starts to lay up after the looper travels about 6.5 inches, or 11.8%..

[BenD]

2 hours ago, Chuck said:

For me the feel of the line is most important after experimenting to find what the correct "walking length" for the head stock is.   And the results are so surprisingly consistent once you repeat them.

Head stock start and end point really is important for consistency. I have measurements on my both ends of my rope walk tracks. It helps to know when to stop the tailstock without constantly checking the rope for how laid up it is.  

 

Now I want to start counting the rotations. Might get some interesting results.