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