ropes and cordages

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ROPES & CORDAGES

PAGE 1

I find this Powerpoint presentation interesting and want to share it.
If, only if, the link is broken try this.

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Just to whet your appetite a bit more here is a slideshow showing that some trees know
about strands and reinforcing fibre strength by using torque to "lay them" click here

All that is observed and comply with in rope making is in order to get the strongest
end-product, cordage, possible.
After cordage has been made the safe way to use it is to understand the why
and how of its fabrication.

It is the only way to understand
--- why cordages are to be handled in such and such manner,
--- why it is important to respect D/d ratio,
--- why/how the strength of the rope is diminished when a knot is made in it.
--- why some knot 'nearly identical' to another one is in fact quite different in
behaviour
All that can only be understood by knowing what is 'inside' the rope .
Here and for a long while I will only deal with cordages that are laid , have
twisted strands.
So if you are interested by that you will have to endure Z/S, helix, and more.
This may appear essentially theoretical, but that is not the best way to see it.

Do you believe that learning about gaze and pressure is purely theoretical to
a scuba diver ? No, this is the very basis of his security.
Nothing theoretical but quite practical matter.

PRE-REQUISITE to be at ease :

Last topic in bats_belfry_4.html :
SIGNING THE CROSSINGS AND LETTERING TOO

in bats_belfry_10.html :
first topic : RIGHT / LEFT
CLOCKWISE / ANTI-CLOCKWISE or COUNTER-CLOCKWISE
WITH THE LAY / AGAINST THE LAY
WITH THE SUN / AGAINST THE SUN
third topic :
WITH THE LAY / AGAINST THE LAY

in bats_belfry_11.html : only topic :
CLOCKWISE / ANTI-CLOCKWISE - COUNTER-CLOCKWISE
with
WITH THE SUN / AGAINST THE SUN

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ABOUT THE TRICK OF USING LETTERS "S" AND "Z" TO FIND THE LAY
OF A CORDAGE

Please! do not make the mistake seen here and there on web sites of using letters Z & S
on cordages that are put in an horizontal position. Doing so you will get the wrong result.
THIS TRICK ONLY WORKS WITH CORDAGES HELD VERTICALLY OR PUT 'IN PERSPECTIVE' as
rail tracks 'receding to the horizon line.'

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HOW TO UNCOIL A ROPE FROM A ROLL COMING OUT OF THE FACTORY. (hence correctly disposed). ONCE AND FOR ALL .

Pre-requisite : bats_belfry N° 4 , N°10, N° 11

A short flash back.

All this 'muddled muddle' about against/with the sun ; with/against the lay ; corkscrew or
counter-corkscrew has hopelessly muddied the water for many.

Here use is made only of 'Z' / 'S' nomenclature and clock-guided orientation .

For a 'Z' or a 'S' laid cordage :
just make sure it will uncoil in such a way that the lay will not be adversely modified.

The following explanation is for a 'Z' cordage.

If you are pulling the inner extremity of it, the one "in the well" you will have to
dispose the whole coiled roll upside down compared to the way you have to dispose it while pulling the outer extremity.

Either way the cordage should progress anti-clockwise in its uncoiling.

Uncoiling from the extremity in the inner well
Uncoiling from the outer extremity

Summary of coiling - uncoiling Z ans S laid cordages
Another summary.

Coiling from center (using sign of crossing)

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HOW TO WIND A CORDAGE ON A REEL

I think that this picture can stand alone.
Now that you know how to use "Z" & "S" and the sign of crossing it should be rather easy
to 'read' the drawing .

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ANOTHER CHANCE TO GET RIGHT THE Z / RIGHT / ANTI-CLOCKWISE
versus S / LEFT / CLOCKWISE OF CORDAGE

Study this picture.
Never again should you have to scratch you head while trying to understand the apparent
contradiction about RIGHT / ANTI-CLOCKWISE and LEFT / CLOCKWISE
Right or Left is the direction of the LINE of the lay
Clockwise or anticlockwise refer to the direction of the ROTATION used to make the rope

Picture above is a courtesy of DENS MODEL SHIPS
I can only give you this good tip, if only for the pleasure of yours eyes, go to :
- their home site
- the rope machine page
- the shop with fabulous models
In case links break : please inform me and do a GOOGLE search with
DENS MODEL SHIPS this will be worth the trouble incurred I assure you.

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FIBRE - THREAD - STRAND - HELIX
Pre-requisite : bats_belfry - 4 , 10 , 11

Helix 'S' et helix 'Z' or 'left' helix and 'right' helix .
Just a short revision of notions exposed in Bat's Belfry_4 about
sign of crossings but this time applied to helices.

Procedure to be followed .

Please make note that result will be correct even if you put the vertical
orientation upside-down. It will still gives the correct answer.
A tip : make use of the angle between the two arrow symbols signs
showing the directions..

Signing of crossing and 'Z' / 'S' are sure tools , even if you put upside-down the vertical
orientation results will not change.
That make derelict the use, to define orientation, of ways full of confusion potential such a :
with/against the sun , (inverse), corkscrew, (anti) clockwise, (in) direct, left/right...

I hold the opinion that it is not quite precise to say, that the strands *are* helices and to
speak about cordage as 'parallel helices cordages', or to stated that these cordages are not
laid or twisted but spun cordage.
Strands follow an approximatively helicidal course but are not really helices except in
modelization.

By the way, while we are there, just as I do not think pedantic to take care not to confuse
'ball' (volume) with 'sphere' (area), I do not think it is pedantic to avoid to confuse
'spiral' (a curve on a plane) with 'helix' (a curve in 3 D) .
It is simply observing a 'guarded' or 'surveyed' language as must be the case when speaking
about technical topics.

Putting apart the straight line parallel to the axis (génératrice in French ) which is the course
asking for the greater energy coming up, the most economical route to go along a cylinder
is the helix. The slope is easier and after one full revolution you arrive directly above your
point of departure.
Squirrels know that and follow such courses.
Fibres, threads, strands will also take the shortest way same under an axial torque :

It is easy to convince yourself :
Flatten a cylinder , decide about the vertical distance to run upward or downward.
Trace a straight line from the chosen corner to the one directly opposite -diagonally -.
Make again the rectangle into a cylinder.
Helix is there. For a cylinder of given height the distance for one full revolution of the helix
is directly dependent of the cylinder diameter.
The greater the diameter the greater the distance.

It follows that for a given length of material before any laying or twisting, there will be after
twisting a greater force applied where the diameter is greater.
What is 'gained' in length is paid for by a diminution of the section (diameter) of the fibres
and by a greater tension existing in them.
Note : the cordage as a whole is shortened.
Pulling tension is some times so high that breakage happen while laying/twisting, the more
so if it is 'hard lay' : this is "loss to the lay"

It must be realized that from the centre to the periphery :
--- either you will have to allow for more matter before commencing, so that after the
twisting motion has been applied all the fibres are under an equal tension whatever be
their position.
--- or you will get a growing gradient of tension from centre to periphery.
The component parts being of equal length at the beginning this length can only be
maintained by applying a greater tension on the outer ones. In the end there will be a
shortening of the 'whole' due to the utilization of available length to do the 'helicing'.

A greater tension in the fibres at the periphery will help to 'compress' those inside.
This is essential in the making of a cordage.
So helices at the centre will be different from those at the periphery and the tension,
or the length, will vary accordingly.

Fibres in a thread, threads in a strand, strands in a hawser, hawsers in a cable, they
will all follow an approximate helicidal course because it is the shortest one : it is also
the one which demand " the less matter for a given tension" or "the less tension for a
given quantity of material" .

This has to be kept in mind as it explain much about laid cordages and their behaviour
when loaded, stretched, the influence of a knot, passage on a pulley...

strand in a real cordage and its helicidal course
...to be continued...

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Copyright 2005 Sept - Charles Hamel / Nautile -
Overall rewriting in August 2006 . Copyright renewed. 2007-2008-2009-2010 -
2011 -(each year)

Url : http://charles.hamel.free.fr/knots-and-cordages/