Steel Wire Rope

Definition

“Wire Rope is made up of several strands of metal wire twisted into a helix forming a composite rope. This pattern is known as laid rope. These wire rope consists of multiple strands.” 

           Wire rope consists of series of coiled wires, which makes “Strand” and those strands coiled around a “Core” (i.e. also a strand exists in the center of wire rope), finally make a wire rope looks like as in above given photo.

Wire: Wires are the main component of wire rope as they are twisted together around a central wire i.e. “Core” to form “Strands” as shown above.

Strand:    The strand is made up of wires which is twisted in a specific manner, as they are helically laid together coiled around of a wire rope.

Core:    The core is the center of wire rope which provides rope stability, is the foundation of a wire rope. Steel core maintained higher resistance and provide strength to the rope.

 Hazards of Steel Wire Ropes

• Inadequate strength of Steel Wire Rope while rigging, hoisting or loading/unloading.
• Inappropriate Bending of wire rope after use (i.e. heavy loads, rollers, drums, etc.).
• Abrasion:  Abrasion (during loading/unloading or dragging by workers) weakens rope by removing metal from both inner and outer wires.
• Vibration (this is because of heavy loads i.e. SWL is 5T and load is 4.8/9T).
• Distortion: Wire rope can be distorted due to high pressure against a sheave, improperly sized grooves or as a result of multi-layer spooling on a drum. Rope with a steel core is more resistant to crushing and distortion. 
  
  
• Corrosion: Rope with a plenty of small wires is more susceptible to corrosion than rope with a small number of large wires. Therefore, if corrosion is expected to have a significant effect on rope performance. The rope may have to be lubricated frequently in service or a galvanized rope may be selected.
• Cabling: Cabling of rope reeving due to block rotation can occur if the rope is incorrectly selected. Applications involving high lifts are particularly vulnerable to this condition, therefore, ropes specifically designed to resist rotation need to be selected.
• Rope length: Wire rope elongate under load. Rope length and/or difference in length between two or more ropes used in a set may be a critical factor and must be considered along with rope selection.

To calculate the Safe Working Load, 

    Square the rope diameter (D) in millimeters (mm) as shown below:

          For Eg: Diameter (D) = 25 mm

          SWL (Kg) = D² (mm) X 8

                          = D (mm) X D (mm) X 8

                          = 25 X 25 X 8 = 5000 kg

    And SWL (T) = 5 tones

Lifting Arrangement

Lifting arrangements of Steel Wire Rope are:

a) Cradle sling:    Cradle sling method are used while lifting coils, steel strip, etc. post SWL check by competent person (i.e. Safety Officer, etc.).

Single basket hitch SWL (when angle is 0-45 degree) 

        = 1.4 X WLL of sling.

• • While lifting boilers and packaging
    cases, etc.

Double basket hitch SWL (when angle is 0-45 degree)

        = 2.1 X WLL of single sling.

[A basket hitch should only be used when the sling is passed through part of the load and the load is balanced on the sling.]

b) Halshing Slings: Method (Double & Choked) using a single sling in place of an endless sling where a ‘bight’ is required.

SWL = 1.6 X WLL of sling.

c) Double Wrap Sling: Double wrap grips the load and helps to prevent it from slipping sideways out of the slings.

Double Wrap Basket Hitch SWL = 2.1 X WLL of single sling.

d) Combination Sling: Timber steel sheets, iron sheets, packing cases, etc. During combination sling lifting the maximum angle between legs should be 90 degrees.

e) Reeving Sling: Lifting tubes, bars, rods, etc.

Double Choke Hitch SWL = 1.4 X WLL of single sling.

Recommendations

1. While using multi-leg assemblies, remember that increasing the angles between the legs will increase the load in each leg. 
   
2. Examine all sling before use and discard defective if any found such.
3. Slings which are found to be unfit for use should be destroyed by cutting them and store them to company’s dumping point.
4. Hooking back to the leg of a sling is not recommended.
5. The Safe Working Load of slings is affected by the method of usage.
6. Check that the crane hook is positioned over the loads center of gravity to prevent swinging when the load is being raised.
7. Correct signals, according to the recognized code, should be given to the crane driver/operator.
8. The signals must be given by the person responsible for the lift and nobody else.
9. Keep wire rope slings away from welding (or any kind of hot work) and flame cutting operations.
10. The law requires that all lifting tackle must be examined by a competent person (i.e. Safety Officer) at regular intervals. This includes the wire rope slings.