Breaking strain by hand

[drjbrennan]

Can anybody tell me, if a string or cord cannot be broken by being pulled by hand; what is it's likely breaking strain above.

In other words, what is the largest force that the average man is likely to exert in the pulling both hands apart motion.

[KnotNow!]

When I played with archery I could pull 75#.  Howard Hill could pull 90#.  This is all day and easily, so the max is somewhat higher.  Of course that posture is one locked arm and the other pulling, using the back and shoulder muscles.  I would think opposed hands would do about as well.  While you are at it use ABOK #143 to break cord in a trick way.  Before I broke many bones in my left hand in a car accident I could break cotton clothes line by this method.  Now I am always afraid I'll hurt the old bones (mostly a fear in my mind not in reality).  I still do #143 and # 142 for thread... but I am a little shy about it.  I'll bet if you use gloves or pile hitch on two handles you can work beyond 100# test.

[nautile]

Hi!
Late post but then I am new here.
First I do not want to play teacher and offend; that being said :
Seems to me there is some confusion here between *isometric* and *isotonic* efforts.
When the "ends" are moving it is *isotonic* and when the "ends" are not moving ( or almost not) it is *isometric*.

It is not being finicky, believe me : in muscular physiology they are worlds apart, as are the possible performances.

When you are drawing your bow that is *isotonic*, when you try breaking a rope by straining it with your hands it is *isometric*.

The laws of the game do not allow you to mix the two. Physiologicaly and "logicalyl"  that would not do.

In strainning the rope by hand you are predominently using muscles that are not in the category that work "against gravity" ( sorry have not the words coming back in English, in French they are called "muscles ( à action) antigravifique".
Muscles used to work constantly against gravity are more powerful.
So you must differentiate
*** between holding the rope more or less horizontaly between your hands (or one end or the rope on a fixed point and the other in your hand)
and
*** more or less verticaly, one end of the rope on a fixed point the other in your hand with the palm of your hand skywards and you trying to close the compass made by your elbow.
In that configuration the tension applied, in all probabilities, would be higher than in the first case (horizontal) since you are using muscles specializing "against gravity".

Hope that I am clear!but none too sure.

Using the "vertical method" you can easily scale your strenght : the higher weight you are able to move up even 2 millimeters, weght attached at the lower end of a rope of very high strenght ( beyond your own strenght).
Over and out!

[KnotNow!]

Hi thanks for the post.  I think I said "Trick Way" to separate *isometric* from *isotonic* or even from momentum.  I think the original post was addressing isometric, which I did by telling of known pulling strenght of several people.  I understood isometric to be pulling against ones own structure.  Then I told of the "trick" where all the rules change.  I did not think of it as confusion but more of an expansion of the original question.  I think that if the muscle group is trained against gravity or against horizontal work only changes the the strenght was aquired, not how it can be applied.  In breaking the rope by inertia the strenght has almost nothing to do with it as the rope shears through itself in interlocked bigthts, so that part of my response is off the original post but fun if you explore it.  In my misspent youth I could easily lift 600# from a sqatting position.  So I could easily break "550 cord" hitched to a ringbolt in the deck.  However I think the orginal question was "hand held line puling against ones own hands."  I envy you, being able to post in a different tongue.  I am so limited that I am not sure of "Bon Jour"!  

[nautile]

Bonjour PABPRES !
Thanks for taking my post as worthy of an answer.

I did not take the trick way on account in my answer since the first post seems to be about " a wild mean for the evaluation of the strenght" so it was an assay and not a conjurer performance.
I was only discussing the notion of "evaluation of the breaking force need when straining by using as measuring tool the force require for drawing a bow"
I have no wish to be polemical as that never give happy results, but  I have the feeling that I have not been clear in my exposition of things.
Pulling strenght in archery is "isotonic" while pulling strenght to break the rope with  2 hands is *isometric" we are aggred but then you cannot infer the isometric strenght of one person if you know his isotonic strenght.

Sure it is the same units used for measuring it ( quantitative identity) but the "qualitative" factor is quite different, just as 10" of rope are as long as 10" of brass rod but you cannot compare the two for that on anything other than the numerical value of the lenght.
For the sake of illustration only : it cannot give you an idea of their dilatation coefficient.
And that is quite comparable with isometric/isotonic, they may be have the same numerals affected to them but their natures are so different that you may not compare them beyond that.
Sure 10 N are 10 N be it isometric or isotonic but what I mean is that by using one instead of the other and individual will obtain quite differents practical results.
If you use antigravity trained muscles ( and that is very dependent of the movement made and even of the hold used ( pronation -supination) you do not obtain the same strenght that you have with the non gravity trained groups.
What I mean is you cannot use the notion of drawing a bow as being the same as the notion of pulling a rope with two hands. The muscular force are quite different in their mechanism therefore in their results.
Hope that it is a wee bit more clearly exposed now, if it is not then too bad I cannot do better in English, and I have the sad notion that I muddled things a bit more.

***Summary :

drawing a bow or straining to elongate a rope between 2 hands may be "structurally" the same gesture but "functionaly" ( physiologicaly, muscle-wise) they are not comparable ( the fibers are not the same, the maximum sustainable duration is not the same, well nothing is the same except that it is the same individual)  the first is isotonic, the last is isometric and for a given individual, even if it seems counter-intuitive, the muscular maximum performance in one is different from the other.
That is why individuals have different  "aptitudes".
Given a global max efficiency the distribution among individuals  of this efficiency is for some *isometric* and for others *isotonic*.
Thanks for your forebearance.
Amicalement.
Charles

[KnotNow!]

Hi Charles, Thanks for the second clarification.  When I post in the international forum I often find that writing in American is not the same as writing in English! :-/  May we return to the original question? Quote from drjbrennan: "In other words, what is the largest force that the average man is likely to exert in the pulling both hands apart motion." All I can think of is examples which do not seem to fit the problem... except of coiurse actually breaking line.  Any suggestions, short of working up through increasing strenghts of line of known value until one finally can't break it?

[nautile]

Hi!
Thanks for having been patient with me.

The method you suggest is ( in my opinion) pragmatically-wise the best one.
Provide the pull is a steady one and not a jerky one.
Provide you leave a recuperation span between each rope as  muscular fatigue will set in.
I am violating my statistical training here but hell we are in a pragmatic assay.
Results should measured on "an average day" to have any practical utility.
Second provision : the gestures done and position adopted while "scaling" should of course the same that will be used "in live situation".

In medical practice we use what we call in French :*dynamometre* dynamometer in English, apparatus that measure the force exerted by the patient.
But that is only window dressing as far as "real" precision is concerned since I have yet to see a verification sheet ( just as for sextant) for the dynomameters I was given to use since 1967!

Medical units specializing in medecine and physiology applied to sports have many "torture racks" to test the athletes patients ( in fact they are not really patient since they are usually in blooming health condition! and characterially they are not always patient either :-).
Just a parting teasing shot ;-): What is for a strong individual an isotonic gesture can by for a less endowed one an isometric gesture.
Before letting the matter rest I want to :
Say thank you  for this friendly chat.
Also I want to say I am quite happy ( even if a bit ashamed when I write a post)  to be on this forum ( learned tons of things) and that I find that knotters here are in fact rather more "weavers". They weave a fine net of human links.

Charles

[drjbrennan]

Just when you think a thread has died.........

By way of clarification. I got some of the comms cord from eBay, and when it arrived tested a small sample and could not break it by hand, also could not find anybody who could.

I thought that this would make it possible for me to state, "This cord has a breaking strain of at least X kilograms force (or Newtons if I was being scientific)" All that remained was to find a figure for 'X', that being the highest average force exerted by a man executing the aforementioned motion.

Have not tried the pile hitch and two handles though.

[Willeke]

Try tying a piece of cord to a high place and a weight on the other end and drop the weight, so you get a shock load. Do several tests with different weights, so you can say which weight it can withstand. Might be better standard than breaking by hand.

Willeke

[Lindsey Philpott]

I have been reading these posts with great interest - the question originally posed seems to have been amended.  When Ashley mentions the breaking of a cord by hand, he does not, as far as I can tell, discuss whether this is dynamic loading or static loading which I, as an engineer, understand, rather than isomateric or isotonic.  I imagine it is dynamic loading ("break the line with a jerk" as Ashley states) because this type of loading is far greater than any that could be applied statically.  Ashley admits to being confused as to whether or not the bowline bend should break at the intersection of the bend or whether it should break at the first disturbance to a straight pull line of strain on the line.  He finds that it normally breaks outside the knot at the first disturbance to a straight-line pull.  This finding is in accord with the principles of physics, in that if a composite material (such as string or cord) is loaded, it will break at the point where it is first disturbed from that straight line pull.  If a slub of cord exists in the line, it will break at that point.  If a local thinning exists, it will break at that point.  For us to determine what is the minimum strain that could be experienced by a line, the answer has to be "It depends".  It depends on whether the line is statically or dynamically loaded, it depends on the perfection or otherwise of the line, it depends on the moisture content surrounding and contained within the line, it depends on the type of line (material), it depends on the loading condition (is it well slack and if so by how much, or is it pulled apart from a stretched position?), it depends on the weave, braiding, lay or other makeup of the line.  It depends also on what is used to do the pulling - are we talking about a mechanics or a mason's hands?  A cook's hands, or a tailor's hands?  How hard do the hands have to be and how far apart are they?  Are they using fingers or palms?  Gloves or not?  How large are the hands?  The questions just go on and on and we somehow never can get to a single solution.  We can only keep asking questions.  Thanks for hearing me out and what a great question!!!

[nautile]

Hello!
My last intervention on this topic unless asked a specific personal question.( and may be for doing  that : off forum would be best.)
I am now transgressing the boring limit but here are some "hard" numbers. But remember : we are *not* a mechanical apparatus and we should not be thought of as being comparable to one, that is a physiological no-no: it is BIOphysics.

Perfectly stated Lindsey Philpott, if I may.

It was just a mental game :
sure, the "scaling" would have been highly individual and with only a very short span of *pragmatical* validity we were all aware of that I think.
No scientific methodology here but a pragmatic, non generalisable "try" ( do not warranted the name assay).

Muscular strength ( untrained, naive, proverbial "average" individual) is very much under most of the rope breaking point. Most would not even begin to really strain a plated cotton masson chalk rope with a 32 kgf / (70.5 pounds if I am not mistaken)  breaking limit.

In a French survey of 92 young men aged 19-20 randomly taken from the general population. The maximal force for elbow flexion in the dominant upper member show that
 95% of population is roughly between 212 N and 353 N that is  that is less than 22kg / 48lbf at  lowest -  36kg /80 lbf at upper limit. ( mean being 283N)

Another study give for uper member *forearm totaly extended from the arm*, held 30° under the horizontal and at 45° ABDuction ( arm opening away from the body axis, and in the body plane) give :
47kg pulling towards the body axis
27 kg pulling upwards and 26 kg pulling downward
24 kg when closing theangle coming toward the body axis
15 kg opening the angle away from the body axis.I have no notion of reprtition of age classes age and size of sample : only that is was in a study on muscles and skelettons disorders afflicting  workers used to heavy and/or repetitive loads in their working hours.

PABPRES and I had lengthy and friendly ( and too me at least stimulating exchanges off the forum. Roy really  has the knack of asking all the good, intelligent, stimulating questions ).
I tried ( operative word, as a sad excuse : this is not my mother tongue here) to state convincingly that  ( biologicaly) muscular strength is a *highly* individual characteristic, with strong nyctemeral variations, with even stronger variations between sexe not to speak of the not negligible variation between the 20y max and the slow decline until 40-45 to reach a max decline at 60-65 ( minus 25-30% what it was at 20).

To make things worse : Inner variance in the individual person performances is quite large and to compound that variance between humans at large  is larger still.

And you are right "impulse", "shock", "momentum" is of course widely different of a steady pull.
You cannot physiologicaly do repeat standardized shocks ( if only for the onset of fatigue). Only steady pulling with no jerking were the point of discussion, as such, without "shock",  there was already a surplus of unmanageable and confounding variables. And it was all to be done "by hands"

As for the physical place where the break will happen in  the rope, sure that is *always* ( not only in the physical domain) where "a difference" exist that things happen.
See our forum :-)

If I may : physiologicaly-wise distance between the hands is  of import  only inasmuch that is make angulation of articulation vary  very much outside a certain angle ( muscle strenght has a bell shape curve with the max at mid elongation of the fibers plus or minus a good tolerance each way). Physiologicaly the position palm upward or downward is of more import.

As I wrote off the forum : your "scaling" will be "valid" till your next sneezing fit or until a  year has elapsed.
And sure your scaling would only applied inasmuch that you place yourself in the same configuration as when getting your personnal "scaling".

Some last words : "where disorder is king, words are the way to it!"

Well you may all breath again.
I am finished.


Cheers all.
Charles

[Dan_Lehman]

Regarding where a rope will break in a knot, there is more conjecture than
evidence; I'm unaware of any good study of this aspect.  There have been
assertions about rope breaking "outside of the knot," which has always
seemed most peculiar to me (and some oddly put deductions that therefore
the knot really didn't weaken the rope!); one person reported tape tied
in a water knot breaking well outside, where the material was being warped
into a u-shaped plane by the compression of the knot's grasp of it--that's a
little easier to understand.

Looking over some broken bits of knotted rope I have, it seems that in some
cases the break occurs later than the SPart's initial deflection (would that
there were super-slow-motion video of the break, or even still photos of the
material/knot at near-rupture which would show the relevant geometry).
I see the initial deflection having some weakening effect, but it might be
moderate, and if a later bend in the SPart is more severe, there could be
sufficient tension delivered there that would lead to it being the rupture
point.

--dl*
====

[roo]

... He finds that it normally breaks outside the knot at the first disturbance to a straight-line pull.  This finding is in accord with the principles of physics, in that if a composite material (such as string or cord) is loaded, it will break at the point where...

Composite?  Unless the rope fibers are immersed in a hardened resin or something, I'd just call string, cord, or rope... well, string, cord or rope.


On breaking points in cordage:
http://tinyurl.com/c4xok

[roy chapman]

This post is going on public.  Shucks and damn.  I tried to keep it private!  So much about my wish to talk about it off the public forum.... Well, the whole bloody post came from someone wanting to know if he could break a string in his hands how strong would it be.  Not where it would break, not about knots, not about bones and muscles... Just how strong can you or me or Tom down the street pull with one part of the cord in his left hand and and one in his right hand.  Pull.  Break.  I suggested about 100#.  This post has gotten so far off thread that it is useless.  I can break cord up to about 100# with no tricks or gimicks.  Go back to the original post.  Charles has added much info about the human body.  Roo, I've tried to understand where your post leads, but please go back  to the first post: "In other words, what is the largest force that the average man is likely to exert in the pulling both hands apart motion."    

[roo]

Roo, I've tried to understand where your post leads, but please go back  to the first post: "In other words, what is the largest force that the average man is likely to exert in the pulling both hands apart motion."    

You're trying to understand too much.  I simply wasn't responding to your question.  I was responding to something else and someone else entirely.