ROPES AND WIRES

ROPES  AND  WIRES

ROPES

Ropes can be made from almost any pliable material,but is generally composed of vegetable fibres i.e. ( natural fibres) ,synthetic fibre,or  steel ,iron or copper wire.

Natural  Fibres Ropes

Consists of 2 types:

1. Hard fibres  -           Manila, Sisal and Coir

2. Soft fibres   -           Hemp,Jute,Flax and Cotton.

1 a)      Manila

Manila is made from Abaca fibre, which comes from  a plant belonging to banana family.Its color varies from ivory white to darkish brown.It is available in various grades, the inner fibres of the leaf being, much finer than those removed from the outer edges.As a rope it is smooth,glossy,strong,flexible,very durable,easy to handle and has a very high resistance to sea water rotting.It is the most common natural fibre in use at sea.It will stretch about 10 – 20 % of its length.

1b)       Sisal

The fibre is taken from Aloe leaves in the pineapple family.Very good sisal has strength equal to medium grade manila and was used when Manila was extremely scarce also it is relatively cheaper then Manila.As a rope, it has an attractive creamy white colour,very brittle,glossy and generally swells more than Manila when wet.It is is not a smooth rope and has hairy surface.

1c)       Coir

It is a hard fibre from the coconut shell after the shell has been water soaked for many months.It is very elastic,red in colour,rough to handle and is extremely resistant to se water rotting.It is principally used as towing and harbour spring.It is about half the weight of Manila and roughly 1/6^th as strong.Its elasticity is about 45 %.

2a)       Hemp

This is a soft fibre which comes from Italy,India,Balkan’s St.Helena and New Zealand.It is a tough fibre of an Asiatic herb viz ‘(Canabis Satira )’.Italian hemp generally regarded as best quality hemp and having strength 1/5^th than top grade Manila.It is largely been superceded by manila and now found at sea around edges of awning and sails,small cordage, high grade twine and wire cores of wire ropes.Also used for making the hand lead line.

2b)       Jute

It is weaken than hemp and is used for the manufacture of  Hessian and cores of the wire rope.

2c)       Flax

It is a long silky fibre from the plant of genus ‘linun’ . It is used for sail and tarpaulin canvas.It is ideal for use as a sewing twine due to its high resistance to abrasion.

2d)       Cotton

It is very soft rope and get easily soiled.It is not strong and rots very easily.At sea it is used as an ornamental rope.

SYNTHETHIC FIBRE ROPES

1.      Polypropylene

It is the lightest synthetic rope.It stretches 40 % but absorbs only 0.1% water and has a low melting point of approximately 165 degree C, highly resistant to acids,alkalis and oils but may be affected by bleaching agents and some industrial solvents.It has wide shipboard use.

2.      Polyester ( Terylene )

Ropes made of this fibre are soft,pliable and immensely strong.They are waterproof and their surfaces dry quickly.They resist acids,oils and organic solvents but are attacked by alkalis.They are stronger than poly propylene.

3.      Polyamide ( Nylon )

This is the strongest of the synthetic fibre.Absorbs very little moisture and has exceptional resistance to substantial loading.It is resistant to alkalis,oils and organic solvents but is attacked by acids.It has great elasticity of approximately 33-40% of its length.

Note: All synthethic fibres are highly resistant to rot and midew.

Preservation Of Fibre Rope ( General )

1.      When removing the rope from a new coil,take the end from the inside so that the turns can be taken off anticlockwise for a right hand rope and clockwise for the left hand rope to avoid disturbing the lay.

2.      Avoid subjecting the ropes to bad nips. ( eg. Sharp edges )

3.      The diameter of the sheaves to be used with the fibre ropes should be at least 10 times the diameter of the rope.

4.      Avoid chafage.

5.      Inspect frequently for excess wear and damage.

Care Of Natural Fibre Ropes

1.     Natural fibres ropes should be stowed away in a well ventilated space and either hung from wooden pegs or  stowed down on gratings.

2.     Do not stow in the vicinity of boilers and other sources of heat excessive heat will cause dryness which makes the fibres brittle.   

3.  The life of manila and sisal is shortened by the action of salt water so after immersion     in sea water,they should be hosed preferably with fresh water.Before stowing away,the ropes should be clean and dry.

3.      Do not surge ropes from drums as far as possible.

Care,Handling and Inspections Of Synthetic Fibre Rope

a)  Special  fibre is not so important with man made fibre ropes but they should not be stowed near boilers or other sources of heat.They should not be covered by tarpaulin when on deck as they can harmed by excessive exposure to sunlight.

b)   Extreme care should be exercised when easing out synthetic ropes from around bitts,cleats or other holding devices to avoid sudden slipping of the line.The low coefficient of friction between synthetic ropes and steel and the large elongation of synthetic ropes under load may cause this sudden slippage.Nylon and polypropylene are particularly critical in this respect.

c)   Due to high stretch of synthetic ropes,large amounts of energy are stored in a line under load.Sudden failure of the rope would cause a snapping back of the line,possibly with resulting casualties to personnel.

d)   Mooring ropes should never be knotted.Knots considerably weaken a rope even after they are removed.

e)   A left-hand laid rope should not be coupled to a right – hand laid rope.

f)   Regular inspection of mooring lines is required.Weak points and potential areas of failure may be noticed,allowing repair or retirement of the rope before it parts in service.

Cautions when using Fibre Ropes

When using a fiber rope under strain, a few number of person as necessary should remain in the vicinity.No one should be allowed to stand in the bight of a rope or across the rope understrain. Synthetic fibre rope unlike natural fibre rope gives no audible warning or visual warning when approaching its breaking point and as the stretch will be recovered almost instaneously when the rope parts there will be considerable recoil.

Strength Of Fibre Rope

The strength of fibre rope is in the following descending order  Nylon, Terylene, Polypropylene, Manila, Hemp, Sisal, Coir, Jute, and Cotton. The term breaking or ultimate strength is the load or weight applied to the material when testing it to destruction.

Splicing a rope reduces its strength by 1/10^th and  knots reduce the strength by ½.The breaking or ultimate strength would depend on the quality of the fibre and processes of manufacture.However given below are formula to estimate the breaking stress of some of the fibre ropes in tonnes.

1.         Manila                         2 x DxD/300

2.         Polypropylene             3 x DxD/300

3.         Terylene                      4 x DxD/300

4.         Nylon                          5 x DxD/300

( D is the Diameter of the rope in mm )

The Safe Working Load ( S.W.L ) may be taken as 1/6^th of the breaking stress.

Example 1:      Given a 40mm polypropylene rope, estimate its breaking stress and the S.W.L?

Breaking Stress = 3xDxD/300 = 3x40x40/300

=  16 tonnes

S.W.L  = Breaking Stress/6   = 16/6

=  2.67 tonne

WIRE ROPES

Wire rope used for marine purposes usually consists of six strands laid up around a central heart of fibre, or about a seventh wire strand Each strand is made up of several wires twisted around a central fibre core or around a single center wire. The number of wires to the strand varies from 7 to 41, the number being governed by size of the rope and also by the purpose for which it is required. Increasing the number of wires to the strand for the same size of rope gives greater strength and flexibility. Wire ropes are referred to by two numbers, the first indicates the number of strands, including a strand which may be used for the central heart, and the second indicates the number of wires to the strand.

Types of Lay

Ordinary Lay

The wires are twisted in the opposite direction to the strands. Right hand rope is normally used ,in which the wires are twisted left handed and the strands are twisted right handed.

Lang’s Lay

The direction of twist of the wires is the same as the direction of the strands.This lay provides a greater wearing surface but should only be used when both ends of the and the load are secured againts rotation. It is not likely to be used for marine.

Guidelines for Handling,Inspection and Removal From Service of Wire Mooring Lines

Handling

-          Prevent kinking of lines.When unreeling the reel should be mounted on a spindle and the line pulled directly off the reel not over the end.

-          If a loop forms, it should be thrown out immediately before any load is placed on the line.

-          The direction of the coiling of the first layer of a line on a plain winch drum depending on the lay of the line.

-          Wire lines should be lubricated periodically. Proper lubrication reduces the abrasive effect of individual wires sliding one another and helps to prevents corrosion. Wire lines are lubricated during manufacture but the lubrication is lost during usage, particularly in marine applications. Ideally the line should be lubricated every two or three months. Several patent varieties of wire line oil are available and the lubricant may be brushed on, or a box lubricator used. Mooring line manufacturer’s recommendations should be followed.

-     The ends of a wire should be periodically reversed, thereby evenly distributing wear on the wire mooring line.

-          When points of wear develop on wire lines, the wire up to and including the sections of wear should be cut off and removed.

Inspection of wire lines and guideline for removal from service

Since wire lines do deteriorate in service, regular inspection is necessary to assess damage to the wire and to perform remedial action. The following guidelines with respect to inspection are presented :

-       The wire diameter should be checked. A marked decrease in wire diameter is sign that the line should be removed from service. Diameter decrease could be indicative of core deterioration, or internal wear and wire failure or internal corrosion.

-       The wires of the outer layer should be inspected for wear and breaks. If lubrication has been good and operating conditions such that the inside wires are intact, the reserve strength of the inside wires is maintained. Thus, only an appropriate strength reduction due to the broken outer layer wires is deducted from the rope strength.

-          Wires should checked for abrasion.If the outside wires show a considerable loss of metallic area, the line should be removed from service.

-          Look for fatigue breaks at or near the valley positions of the strands. If one fatigue break of this type is noted, the line should be removed from service, since further deterioration of this type has doubtlessly taken place where it cannot be easily detected.

Recommended Construction Of Wire Ropes in Marine Use

For stranding rigging  ( Stay, Shrouds , Preventer etc. )

S.W.R.            7 x 7   up to rope diameter 28 mm

S.W.R.            7 x 19 up to rope diameter  32 – 48 mm

S.W.R 7x 37  for rope diameter over  52mm

For Cargo Handling

F.S.W.R          6x 24   ( in some cases like cranes, ESFSWR )           6x37 may be used.

For Mooring Ropes

F.S.W.R.         6x37    for general use with powered winches ; 6x36 ropes with wire care should be used for rope size up to 40 mm .

For Boat Falls

( 17x7 ) or ESFSWR  ( 6x36 ) for rope sizes up to 16mm and 6x36 for larger sizes.

Cargo Lashings

F.S.W.R. ( 6x 12 ) for size 8 – 16mm and  6x24 for larger sizes.

Strength Of Wire Ropes

The breaking stress of Wire Ropes in tonnes is given approximately by the following formula:

1.                  For Wire Rope of  ( 6 x 12 )  =  15 x D x D / 500

2.                  For Wire Rope of  ( 6 x 24 )  =  20 x D x D / 500

3.                  For Wire Rope of  ( 6 x 37 )  =  21 x D x D / 500

4.                  And the Safe Working Load = 1/6 of the breaking stress.

Example:           Given a Wire Rope of  ( 6 x 12 ) , Diameter = 20mm: Find the breaking stress and the S.W.L for the Wire Rope.

            Breaking Stress  =       15 x 20 x 20 / 500  = 15 x 400 /500    =          12 tonnes.

            S.W. L                =1 / 6 x Breaking Stress       =          12 / 6   =          2  tonnes.

Assignment :

1.                   Calculate the breaking stress and Safe working load , atleast for 3 different type of wires or ropes.

2.                  Calculate the breaking stress and S.W.L for the following ropes and wires.

a.            Given a 40mm diameter polypropylene rope, estimate its breaking stress and safe working load.

b.                  Find the size of the smallest manila rope suitable for a load of 2 tonnes.

c.                   Find the breaking stress and safe working load of 6 x 24 , 24mm diameter wire rope.

d.                  Find the breaking stress and safe working load of 6 x 37 , 20 mm diameter wire rope.