ROPE CONSTRUCTION

 

    PARALLEL YARN

 

    PARALLEL STRAND

 

    LAID

 

    WIRE ROPE

       OR STRANDED

 

    BRAIDED

 

    PLAITED

 

    OVERVIEW

 

 

         

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

ROPE CONSTRUCTION

 

 

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 Strand

  Laid

  Wire Rope or Stranded

  Braided

  Plaited

 

 

 

PARALLEL YARN

 

 

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.

 

 

 

 

 

 

PARALLEL STRAND

 

 

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.

 

 

 

 

 

 

LAID

 

 

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.

 

 

 

 

 

 

WIRE ROPE OR STRANDED

 

 

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.

 

 

 

 

 

 

BRAIDED

 

 

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.

 

 

 

 

 

 

PLAITED

 

 

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.

 

 

 

 

 

 

OVERVIEW

 

 

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.

 

ROPE CONSTRUCTION

TENSION-

TENSION

STRENGTH

TO DIAMETER

TOUGHNESS

CYCLING OVER

SHEAVES

PARALLEL YARN

VERY GOOD

EXCELLENT

POOR

POOR

PARALLEL STRAND

VERY GOOD

HIGH

MODERATE

MODERATE

LAID (3 OR 4 STRAND)

GOOD

MEDIUM

GOOD

HIGH

WIRE ROPE CONSTRUCTIONS

VERY GOOD

HIGH

GOOD

EXCELLENT

BRAIDED

GOOD

MEDIUM

VERY HIGH

MEDIUM

PLAITED

GOOD

MEDIUM

EXCELLENT

GOOD

 

 

 

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