Engineering materials: How are wire ropes constructed and selected for lifting applications, and how is the factor of safety determined under Australian standards?
Describe the construction and properties of steel wire ropes, calculate the safe working load from minimum breaking load and a factor of safety, and identify inspection requirements under AS1418
A focused answer to the HSC Engineering Studies Lifting Devices dot point on wire ropes. Strand construction, lay direction, minimum breaking load, factor of safety under AS1418, retirement criteria, the Port Botany shipping container crane example, and worked HSC-style past exam questions.
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What this dot point is asking
NESA wants you to describe how steel wire rope is constructed, calculate safe working load using a factor of safety, identify the rope retirement criteria, and link these to the Australian Standards governing lifting (AS1418 for the crane, AS2759 for rope selection, care and use).
The answer
Construction of steel wire rope
A steel wire rope is built from three nested elements:
- Wires. High-tensile carbon steel wires, 0.3 to 4 mm diameter, drawn to typical tensile strength of 1770 MPa or 1960 MPa.
- Strands. Wires laid helically around a centre wire. A 6 x 19 construction has 6 strands of 19 wires each. A 6 x 36 has 6 strands of 36 wires each, providing better flexibility from the higher wire count.
- Core. Either a fibre core (FC, polypropylene or natural fibre) for flexibility, or an independent wire rope core (IWRC) for higher strength and crushing resistance.
Lay direction
The strands are laid around the core; the wires are laid around their strand. Each can be right-hand or left-hand lay.
- Ordinary (regular) lay. Wires and strands lay in opposite directions. The wires appear to run parallel to the rope axis on the surface. More resistant to crushing and untwisting.
- Lang's lay. Wires and strands lay in the same direction. The wires appear at an angle on the surface. More flexible, better fatigue life over sheaves, but tends to untwist; only used when both ends are restrained.
Minimum breaking load and factor of safety
The minimum breaking load (MBL) is the load at which the rope, in new condition, will fail by tensile fracture. It is given on the rope manufacturer's test certificate.
The safe working load (SWL) is:
The factor of safety required by AS1418 depends on the duty:
| Application | Factor of safety |
|---|---|
| Manual hoist | 4 |
| Powered crane hoist | 5 |
| Personnel lift | 12 |
| Mine winder (with people) | 10 |
The high factor for personnel is to allow for shock loading, dynamic effects and wear between scheduled inspections.
Australian standards
- AS1418.1: General requirements for cranes and hoists.
- AS1418.4: Tower cranes.
- AS1418.5: Mobile cranes.
- AS2759: Steel wire ropes for use in lifting applications. Includes selection, installation, inspection and discard criteria.
- AS1735: Lifts, escalators and moving walks (passenger).
Inspection and retirement
Steel wire ropes wear progressively. Routine inspection looks for:
- Broken outer wires (count over one lay length, the distance for one full helical turn of a strand)
- Reduction in diameter
- Corrosion
- Localised damage from kinking, crushing or birdcaging
- Heat damage and lubricant loss
- Wear on terminations and end fittings
A rope is removed from service when any criterion in AS2759 is exceeded. Routine inspections are at least six-monthly; rigorous inspections are annual.
Wire rope on a typical lifting device
A tower crane on a Sydney CBD building site might have 19 mm or 22 mm diameter 6 x 36 IWRC rope on the main hoist drum. The rope passes through the boom-tip sheaves and the load block (typically 2-fall or 4-fall, providing additional mechanical advantage as discussed in the pulleys dot point) and terminates at a wedge socket or thimble.
Ship-to-shore container cranes at Port Botany use rope diameters up to 38 mm with IWRC construction and lubricated steel sheaves. The rope is rotated end-for-end periodically to even out wear.
Past exam questions, worked
Real questions from past NESA papers on this dot point, with our answer explainer.
2023 HSC style4 marksA tower crane hoist uses 6 x 36 IWRC wire rope of 19 mm nominal diameter. The minimum breaking load is 215 kN. AS1418 requires a factor of safety of 5 for crane hoist ropes. (a) Calculate the safe working load (SWL). (b) Identify two visual inspection criteria for retirement of the rope from service.Show worked answer →
(a) Safe working load.
The rope can safely carry a 43 kN (about 4.4 tonne) static load, with a fivefold reserve before failure.
(b) Retirement criteria under AS2759 and AS1418. Two of the following:
- Broken wires. Six randomly distributed broken outer wires in one lay length, or three broken wires in one strand in one lay length, trigger replacement. Wire ropes typically show progressive surface wire breakage before total failure.
- Diameter reduction. A reduction of 7 percent or more from the nominal diameter, due to wear, crushing or core deterioration, requires replacement.
- Corrosion. Heavy external corrosion or any sign of internal corrosion (rust dust between strands) is a retirement trigger.
- Kinking, birdcaging or other deformation. Any permanent deformation that changes the cross-section is grounds for immediate retirement.
- Heat damage. Discolouration from welding spatter or fire is grounds for retirement (high-strength wires lose tensile capacity above 200 degrees C).
Inspections are scheduled (six-monthly minimum) and recorded in the crane log book. The crane cannot be used until a competent person has cleared each issue or fitted a new rope.
Markers reward (1) the SWL formula with correct factor of safety, (2) the unit conversion of breaking load and SWL, and (3) two specific retirement criteria with quantitative thresholds where applicable.
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