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► ROPE CONSTRUCTION ► PARALLEL YARN ► PARALLEL ► LAID ► WIRE ROPE OR STRANDED ► BRAIDED ► PLAITED ► OVERVIEW
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Synthetic fiber
ropes have different rope constructions, but in general a synthetic fiber rope is build up of strands. A strand is made up of yarns. A
yarn is made up of a very large number of fine fiber
filaments. A fiber filament has a diameter of 12 microns. The advantage of
synthetic fibers is that synthetic fibers can tolerate substantial more twist in contrast
with metal wires. The rope construction is
important for the strength, elongation, shape stability and more.
In general there are 6 broad rope
construction categories for synthetic fiber ropes.
The rope construction groups are listed below: ► Parallel Yarn ► Parallel ► Laid ► Wire Rope or Stranded ► Braided ► Plaited
Parallel
yarn is the most common parallel lay rope construction. The parallel yarn is
made up with parallel yarns. The yarns
are parallel to the axis of the rope. The parallel yarns are bound with an
extruded polyethylene jacket. The
advantage of the parallel yarn rope construction is that the rope
construction has high strength, high modulus and it
does not rotate under severe load.
The parallel strand is made up of
(laid or braided) strands and the strands are assembled together under an
outer braided fiber jacket. The strands are parallel with the axis of the rope. The
advantage of the parallel strand rope construction is that the rope construction has
high strength, high modulus and it does not rotate under severe load. The parallel strand rope
construction is recommended for offshore mooring lines.
The laid rope construction also
known as twisted rope is constructed with a twist of 30 degrees of more. The twists give the rope a
structural integrity and there is no need for external jackets to bind the
strands together. In general the laid rope
construction has 3 and 4 strands. The advantages of the laid rope
construction are that the rope construction has a medium strength and there is no
need for a jacket. The disadvantages of laid construction are that the laid
rope construction is not used for large ropes and for high modulus fibers and it does rotate under severe load.
The wire rope construction (WRC) also known as the stranded rope construction consists of twisted strands in one or more concentric rings around a core. It is the same rope construction as the metal wire rope. By synthetic fiber ropes the core can carry some loads, but in general the core of the wire rope is not designed to carry load. If the core is designed to take a carry load the core becomes a strand. The simplest form of wire rope construction is the spiral or bridge strand. The spiral strand is a single large strand. The good wire rope constructions are 6 strands around the core and twelve strands around six strands around the core. Too much strands make that the wire rope cannot tolerate torque anymore. The common wire rope constructions are assembled with a nylon jacket or braided polyester jacket. To make the jacket extremely tough. A jacket can be made of
a combination of both a braid and a coating or an
extrusion. The advantages of wire rope constructions are good strength, good
modulus and good fatigue. The disadvantages of wire rope constructions are
that wire ropes will rotate under tension and the wire rope is not good
tolerant to torque. A rotation can upset the balance of loads between outer
and inner strand layers. The wire rope construction can be
made torque balanced by dividing the wire rope in two or more concentric
layers. One strand layer is assembled in
one direction and the other strand layer is manufactured in the opposite
direction. The direction of the twist is known as the lay of the rope. A wire rope construction can be twisted clockwise called right-laid or Z-laid or counter clockwise called left-laid or S-laid. Most wire rope constructions are Z-laid. The
twist direction of the fibers, yarns and strands is
important for the strength and the shape of the wire rope.
The braided rope construction can
be either circular or solid. The braided rope construction consists of an
equal number of interwoven S-laid and Z-laid strands. The common braid rope
construction patterns are over 2 over 2 or 1 under 1. Each strand is constructed of one
or more parallel twister yarns. The outside yarns of the braids should be
parallel with the axis of the rope. There are different
braided rope constructions. The hollow braid also known as single circular
braid and maypole braid has no core. The hollow braid or single circular
braid is not used for large ropes. Solid braid or parallel braid has a braided
sheath over a core. Solid braid is very strong. Braid on braid also known as
double braid has a braided core inside a braided sheath. In fact two braided ropes combined
into one rope. The braid on braid rope construction has less flexibility and
less stretch than a hollow braid rope construction. The braid on braid rope
construction is used for offshore mooring lines. The common braid on braid rope construction is 12 strand
braided rope. The advantages of braided rope construction are that it is
torque free or it does not tend to rotate under
tension, it has moderate strength, moderate modulus and a good structural
integrity. The disadvantages of braided rope
construction are that the twist affects the strength, the rope construction
is not torque tolerant,the rope construction needs larger
diameter for the same strength and braided rope constructions cannot be made
of high modulus fibers.
The plaited rope construction is also known as square braids. The braid rope construction has 8 or 12 strands of square cross sections. The advantages of plaited rope construction are that it is torque free or it does not tend to rotate under tension, it has moderate strength, moderate modulus and a good structural integrity. The disadvantages of plaited rope construction are that braided rope constructions cannot be made of high modulus fibers and the rope construction needs larger diameter for the same strength. A larger diameter can give problems with drag and vortex shedding.
The plaited rope construction is
also known as square braids. The braid rope construction has 8 or 12 strands
of square cross sections. The advantages of plaited rope construction are
that it is torque free or it does not tend to rotate under tension, it has
moderate strength, moderate modulus and a good structural integrity. The
disadvantages of plaited rope construction are that braided rope
constructions cannot be made of high modulus fibers
and the rope construction needs larger diameter for the same strength. A larger diameter can give
problems with drag and vortex shedding.
A
comparison of the properties of the rope constructions are given in the table
below. In the
table the rope constructions are compared on the rope properties:
tension-tension, strength, toughness and cycling over sheaves.
The table can be used as guide for the selection of the rope construction. For
permanent mooring is the cycling over
sheaves not important. The rope construction and yarns are important for the
rope, but the material of the fibers is also very important.
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