Steel wires for wire ropes are made of non-alloy carbon steel. They possess high strength to enable the ropes withstand large tensile forces.
Construction of a wire rope
In case of stranded ropes several strands laid helically around a core. This core can be:
- Fiber core, generally made up of natural fibers like sisal. Natural fibers are also used but they absorb less lubricant, though, are stronger. Natural fiber can absorb up to 15% of their weight in lubricant and so protect the inner wires much better from corrosion than synthetic fibers do.
- Wire strand core, is made up of one additional strand of wire. Following picture shows rope made up of six strands wrapped around a core made up of the same strand. This rope has 6 strands each made of 7 wires and so is called a 7 x 7 rope.
3. Independent wire rope core (IWRC): These are the most durable in all types of environments. Core is itself is a wire rope made up of wires and strands.
How to Measure the Size of a Rope
The size of a wire rope is the diameter in millimetres of a true circle, which will just enclose all the strands. Measure 3 places at least 2m apart. The average is to be taken as the diameter of the rope.
Multi-strand ropes are all more or less resistant to rotation and have at least two layers of strands laid helically around a centre. The direction of the outer strands is opposite to that of the underlying strand layers.
Inspecting Steel Wire Rope for defects
The following defects or damage may be spotted in a wire rope:
- Distortion of Rope or Strands can be the result of damage by kinking or crushing or sharp bending.
- Flattening of Some of the Outer Wires by Abrasion: Easily spotted because the abrasion gives the flattened wires a bright and polished appearance, but they do not affect as such. Flats, which extend to three-quarters of the diameter of the wires will reduce their cross-sections and therefore their individual strengths slightly. The loss in strength of the whole rope will be very small. These flats are less serious than flattening of whole rope.
- Broken Wires: These are usually the result of fatigue and wear, and mostly occur in crane wires or wire ropes at the derrick heel block where the rope exits obliquely. A broke wire indicates damage due some reason. Investigation must be made to find more damage.
- Corrosion: The action of damp and salt on the wires from which the galvanising has worn off, if this occurs to the inner wires first it causes rust to fall out of the rope and is therefore easily detected. The action of fumes and funnel gases, which attack the outside wires, the effect then becomes visible on inspection. Contact with acid, which soaks into the heart and attacks the inside wires; this is not necessarily noticeable on the outside of the rope, and can be the cause of parting without warning.
Lack of lubrication is a frequent cause of corrosion. When a wire rope is under tension it stretches and becomes thinner, and during this process the individual wires are compressed and friction is set up; the fibre heart and cores are also compressed, releasing oil to overcome the friction. A wire rope of outwardly good appearance, but with a dry powdery heart or core, has not been properly maintained and should be treated with caution.
- Effect of Extreme Cold: When subjected to extreme cold a wire rope may become brittle and lose its flexibility, and an apparently sound rope may part without warning. The brittleness is not permanent and the rope will regain its resilience in a normal temperature, but the potential danger should be remembered when working wires in very cold climates.
Testing of Steel Wire Rope
After manufacture of each production length of rope, test samples are cut from the finished rope and strand. These samples are used for following tests:
- Tensile test to destruction.
- Tests of performance.
- Test for torsion.
- Test for quality of galvanising.
Each coil of wire is accompanied by a certificate of conformity and a test certificate showing the minimum breaking strength of the wire.
Various discard criteria
Though, a popular convention being visible broken wires, other abnormalities also may be considered as warning signs for a discard. At least these signs can be pointers for further investigation of seeking of expert advice.
- Broken Wires: This is the most commonly used discard criteria. ISO 4309:2011 list the number of breaks in the load bearing wires in outer strands needed to determine the discard criteria. The number of visible broken wires are able to be counted in that area of the rope where most of them are present. The length of the area has been determined to be 6d or 30d (“d” = nominal rope diameter). The rope must be discarded whenever the number of broken wires counted, in 6d or 30d, is equal to, or greater than, those listed in the standards.
- Diameter: When the over all diameter of the rope is reduced by 8 % or the outer wires have lost it’s diameter by 33 % through wear. This reduction in diameter of the rope is measured on a portion of the rope which has been subjected to abrasion and stresses.
- Popular convention: When the total number of visible broken wires exceed 10 % of the total number of wires in the rope in a length equivalent to eight times the diameter of the rope.
- Plastic damage: When there is evidence of considerable plastic wear or surface enbrittlement.
- Sudden reduction: When the diameter of the rope has suddenly reduced or the lay length has suddenly increased or decreased .The decrease in lay length is usually associated with waviness in the rope.
- Corrosion: When there is evidence of severe corrosion like chain pitting. The condition may be alarming if in addition to corrosion, fatigue is also present.
- Evidence of internal corrosion: This can be recognized by slackness of outer wires due to the internal wires corroding away or under wet conditions, a rusty emulsified liquid /substance exude between the strands under load.
- Localised distortion: When the rope has been subjected to localized distortions as a result of mechanical damage, crushing, kink, bird cage, etc.
- Collapse of core: When the core of the rope has collapsed. If likely to cause the strands to bear unequal stresses they must be considered as reducing the strength of the rope considerably and should they be sufficiently serious to cause the heart to protrude, the rope must be discarded.
- Excessive exposure to heat:When the rope has been subjected to a high temperature or heat due to fire
- Shock: When the rope has been subjected to sever shock load or over load due to some accident with the equipment.
- Abrasion caused damage: Abrasion can simply be caused by the constant passing of the wire rope over the drum, or the sheaves in the application of work. However, improper lubrication during service of the wire rope will accelerate the affects of abrasion as wires begin to stick together.
- Broken Strand: When a strand breaks, the rope must be immediately discarded.
Lubrication during the manufacturing process
If the rope has a fiber core center, the fiber will be lubricated with a mineral oil or petrolatum type lubricant. The core will absorb the lubricant and function as a reservoir for prolonged lubrication while in service. If the rope has a steel core, the lubricant (both oil and grease type) is pumped in a stream just ahead of the die that twists the wires into a strand. This allows complete coverage of all wires.
1. Penetrating lubricants: contain a petroleum solvent that carries the lubricant into the core of the wire rope then evaporates, leaving behind a heavy lubricating film to protect and lubricate each strand.
2. Coating lubricants: penetrate slightly, sealing the outside of the cable from moisture and reducing wear and corrosion from contact with external bodies.
3. Combination: Most wire ropes fail from the inside, it is important to make sure that the center core receives sufficient lubricant. A combination approach in which a penetrating lubricant is used to saturate the core, followed with a coating to seal and protect the outer surface, is recommended.
- Petrolatum compounds, with the proper additives, provide excellent corrosion and water resistance. They are translucent, allowing the technician to perform visible inspection. These lubricants may drip off at higher temperatures but maintain their consistency well under cold temperature conditions.
- Asphaltic compounds generally dry to a very dark hardened surface, which makes inspection difficult. They adhere well for extended long-term storage but will crack and become brittle in cold climates. These compounds are the coating type.
- Various types of greases are used for wire rope lubrication. These are the coating types that penetrate partially but usually do not saturate the rope core. Common grease thickeners include sodium, lithium, lithium complex and aluminum complex soaps. Greases used for this application generally have a soft semifluid consistency. They coat and achieve partial penetration if applied with pressure lubricators.
- Petroleum and vegetable oils penetrate best and are the easiest to apply because proper additive design of these penetrating types gives them excellent wear and corrosion resistance. The fluid property of oil type lubricants helps to wash the rope to remove abrasive external contaminants.
Many lubricants available in market are unleaded environmental types. The coating is non-sheening and found excellent for resistance to softening under severe working conditions. They are water resistant and usable for extreme pressure situations too., With good quality lubricants the viscosity and slickness stay alive through the Wire Rope as the strands move during load-bearing operations. This attribute helps with continuous redistribution to cover any internal strand areas that may have been missed during the initial lubrication process.
A good medium would have sound structural and chemical stability. Pressure additive provides exceptional protection against wear and shock loads. The additives are put to enhance resistance to water wash-off, protection against fling-off at high Wire Rope travel speeds, and long service in a high- temperature environment. Non corrosiveness with steel and copper bearing alloys and compatibility with conventional seal materials are the usual properties. Lubricants nowadays, come with bio-aquatic toxicity tests certificates. The copy, alternately, is available upon request.
Practical considerations while lubricating
There is difference in overhauling a wire rope that is already being maintained under PMS and a rope that is maintained when the situation demands. A rope that is secured locationwise and well lubricated will seldom go bad.
- To remove the dirt and any layer of hardened lubricant or other contaminant, rope must be cleaned with a wire brush and petroleum solvent, compressed air or steam cleaner before relubrication. The wire rope must then be dried and lubricated immediately to prevent rusting.
- Modern lubricators with machine intervention is the current trend of systematic application.
- Field lubricants can be applied by spray, brush, dip, drip or pressure boot. Lubricants are best applied at a drum or sheave where the rope strands have a tendency to separate slightly due to bending to facilitate maximum penetration to the core. If a pressure boot application is used, the lubricant is applied to the rope under slight tension in a straight condition.
- Excessive lubricant application should be avoided to prevent safety hazards.
- Wire rope must never be handled with bare hands.
Care and Maintenance of Steel Wire Rope
- Wire ropes have a lubricant incorporated during manufacture. This serves a dual purpose;
1. it provides corrosion protection; and
2. it minimises internal friction.
- The protection provided by this manufacturing lubricant is not enough for ongoing maintenance later. The needs supplement during service period. This service lubricant is termed the ‘dressing’. Dressing would depend on the type of rope and its usage.
- Details of the maintenance of steel wire rope carried, or fitted in, ships should be laid down in the Planned Maintenance Schedule (PMS). Reference must be taken from manufacturer’s guidelines.
Protecting Wire ropes
- Wire rope can be damaged by worn sheaves, improper winding and splicing practices, and improper storage.
- High stress loading, shock loading, jerking heavy loads or rapid acceleration or deceleration (speed of the cable stopping and starting) will accelerate the wear rate.
- Corrosion can cause shortened rope life due to metal loss, pitting and stress risers from pitting.
- If a machine is to be shut down for an extended period, the rope should be removed, cleaned, lubricated and properly stored.
- In service, corrosion and oxidation are caused by fumes, acids, salt brines, sulfur, gases, salt air, humidity and are accelerated by elevated temperatures. Proper and adequate lubricant application in the field can reduce corrosive attack of the cable.
- Abrasive wear occurs on the inside and outside of wire ropes. Individual strands inside the rope move and rub against one another during normal operation, creating internal two-body abrasive wear. The outside of the cable accumulates dirt and contaminants from sheaves and drums. This causes three-body abrasive wear, which erodes the outer wires and strands. Abrasive wear usually reduces rope diameter and can result in core failure and internal wire breakage. Penetrating wire rope lubricants reduce abrasive wear inside the rope and also wash off the external surfaces to remove contaminants and dirt.
- Wire hawsers should be stowed on reels under a fitted cover whenever possible. The surface of a wire hawser should be washed with fresh water to free it from salt, then dried with cloths and lightly smeared with the appropriate lubricant.
- Compatibility with sheave
I. Size of Sheave Required for a Wire Rope Hoist. The diameter of sheave required for each type of six-strand wire rope supplied should be at least twenty times the diameter of the wire. The diameter of a sheave used for any wire rope will considerably affect the life of that rope. As the rope bends round a sheave the strands and wires farthest from the centre of curvature move apart and those nearest the centre of curvature move closer together. This results in the generation of considerable friction between these wires and strands, and the smaller the sheave the greater will be the friction.
II. Friction also increases rapidly with the speed at which the rope is moving. While the rope is bent round a sheave the outer wires are also subjected to a marked additional stress, and the smaller the diameter of the sheave the greater will be the stress. For these reasons the minimum diameters of sheaves recommended from practical experience for various types of ropes at speeds not exceeding 60m per minute are 20 times the diameters of the ropes. For increase in speed figures are higher. This will give a rope a reasonable life, but it is emphasized that its life will be greatly increased if still larger sheaves are used.
III. The life of a rope used for hoisting can also be considerably shortened by using the wrong type of sheave. The groove in the sheave must fit and support the rope as it travels round the sheave, otherwise there will be increased internal friction and external wear.
IV. A sheave with too wide a groove, will result in a flattening of the rope and considerable distortion and internal friction.
V. A sheave with too narrow a groove, results in the rope not being supported, the wires of the strands being subjected to considerable wear, and friction being set up between the rope and the sides of the groove.
VI. The groove of the correct sheave should be shaped in cross-section to the true arc of a circle for a distance equal to one-third of the circumference of the rope, and the radius of the groove should be between 5 and 10% greater than the specified radius of the rope.
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