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Wire Ropes

Wire Ropes
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
Stranded ropes
In case of stranded ropes several strands laid helically around a core. This core can be:

  1. 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.
  2. 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:

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.

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:

  1. Tensile test to destruction.
  2. Tests of performance.
  3. Test for torsion.
  4. 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.

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.

Methods
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.

Constituents

  1. 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.
  2. 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.
  3. 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.
  4. 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.

  1. 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.
  2. Modern lubricators with machine intervention is the current trend of systematic application.
  3. 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.
  4. Excessive lubricant application should be avoided to prevent safety hazards.
  5. Wire rope must never be handled with bare hands.

Care and Maintenance of Steel Wire Rope

Protecting Wire ropes

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