Wire rope and factors of safety: HSC Engineering Studies Lifting Devices
“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”
Steel wire rope is built from helically laid strands of high-tensile wires around a fibre or wire core. Safe working load is the minimum breaking load divided by a factor of safety from AS1418 (typically 5 for crane hoists, 12 for passenger lifts). With multi-fall reeving, multiply a single rope's SWL by the number of falls to get the total load capacity. Retire ropes when broken-wire counts, diameter reduction, corrosion or deformation exceed AS2759 thresholds.
A wire rope is basically a very strong, flexible cable made of hundreds of thin steel wires twisted together, first into bundles called strands, then the strands twisted around a central core, a bit like how a thick rope of hair is really lots of thin strands wound together. You never load a lifting rope right up to the point where it would actually snap. Instead, engineers find the load at which a brand-new rope truly breaks (the minimum breaking load), then divide it down by a safety margin called the factor of safety, so the rope only ever has to carry a small fraction of what it could ultimately handle. That margin is bigger for things that could kill someone if they failed, like a passenger lift (divide by 12), and smaller for things like a hand-cranked hoist lifting materials (divide by 4). On top of that, because a wire rope wears out gradually rather than snapping without warning, inspectors count broken wires and check the rope's diameter regularly and simply retire it once it crosses a defined wear limit, long before it gets anywhere near actual failure.
Note: Lifting Devices is a Preliminary (Year 11) module of NSW Engineering Studies, not an HSC module. The four HSC modules are Civil Structures, Personal and Public Transport, Aeronautical Engineering, and Telecommunications Engineering. This page is kept as Preliminary reference; it is not assessed in the HSC Engineering Studies exam.
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.
- MBL (minimum breaking load) is fixed for a given rope, from the manufacturer's test certificate; it does not change with how the rope is used.
- FoS (factor of safety) is set by AS1418 according to the application and its consequence of failure, not by the rope itself: 4 for a manual hoist, 5 for a powered crane hoist, 10 for a mine winder carrying people, 12 for a personnel lift.
- The same physical rope therefore has a different SWL depending on what it is installed on; always check which FoS the application requires before dividing.
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.
When a question mentions "2-fall" or "4-fall" reeving, the load is shared between that many parallel rope segments. Work out the SWL of ONE rope segment first (), then multiply by the number of falls to get the total load the hook or cage may carry. Do the reverse when you are given a total hook load and asked for the tension in a single rope segment: divide by the number of falls before comparing against SWL or MBL. Always state which value, the single-segment SWL or the total system SWL, you are quoting; markers cannot award marks for an unlabelled number.
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.
A mobile crane uses 16 mm 6 x 19 IWRC rope with MBL = 150 kN. Required factor of safety 5.
SWL = 150 / 5 = 30 kN.
If the load is suspended from a 4-fall reeving (4 rope segments supporting the load), each rope segment carries one quarter of the load. The maximum load on the hook is kN, equivalent to about 12.2 tonnes. The hook block, pin and any spreader must also be rated to at least 120 kN.
Marker's note: state the single-segment SWL first, then explicitly multiply by the number of falls; markers cannot infer which figure is which if only the final answer is shown.
- Calculating SWL on a single rope when the lift uses multiple falls
- With 4-fall reeving, each rope segment carries 1/4 of the load, so the SWL on the hook is 4 times the rope's SWL. The calculation depends on the reeving arrangement.
- Using ultimate tensile strength of one wire instead of MBL of the rope
- MBL is the rope's measured breaking load, not the sum of wire strengths (which would over-estimate by 5 to 15 percent due to wire interaction).
- Forgetting dynamic effects
- SWL is a static value. Shock loads from a stalled lift or a snatched start can momentarily double the rope tension. The factor of safety is partly to cover this.
- Using rope past retirement
- Visual inspection criteria are strict because partial rope failure is not visually obvious. Lay length and broken wire counts must be measured.
- Assuming a bigger factor of safety means a "safer" rope in isolation
- FoS is set by the application's consequence of failure, not by the rope's quality; a personnel lift's FoS = 12 does not mean the rope itself is superior to a crane hoist rope with FoS = 5.
Exam-style questions
Questions in the style of NESA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
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.
Practice questions
Original practice questions graded from foundation to exam level, each with a full worked solution. Try them before revealing the solution.
foundation3 marksA manual chain hoist uses wire rope with a minimum breaking load (MBL) of 96 kN. AS1418 requires a factor of safety of 4 for a manual hoist. Calculate the safe working load (SWL), in kN.Show worked solution →
Marking criteria: 1 mark for correctly identifying FoS = 4 for a manual hoist, 1 mark for correct substitution into the SWL formula, 1 mark for the correct answer with units (24 kN).
foundation3 marksState two structural differences between a wire rope with a fibre core (FC) and one with an independent wire rope core (IWRC), and explain why a crane manufacturer would specify IWRC for a heavy tower crane hoist rope.Show worked solution →
Two structural differences: (1) FC uses a polypropylene or natural fibre rope as the core, while IWRC uses a small wire rope as the core. (2) IWRC gives greater crush resistance and a higher overall breaking strength for the same outer diameter, while FC is lighter and more flexible but compresses more easily under load.
A heavy tower crane hoist rope is wound tightly onto a drum and repeatedly loaded at high tension, which crushes a fibre core over time and reduces support for the outer strands. IWRC resists this crushing and better maintains the rope's round cross-section, so manufacturers specify it for high-load, high-cycle applications such as tower crane hoists.
Marking criteria: 1 mark per correct structural difference (to a maximum of 2), 1 mark for a correct engineering reason linking IWRC to crush resistance under sustained heavy loading.
core5 marksThe table below gives the AS1418 factor of safety (FoS) for four lifting applications.
| Application | FoS |
|---|---|
| Manual hoist | 4 |
| Powered crane hoist | 5 |
| Personnel lift | 12 |
| Mine winder (with people) | 10 |
A wire rope with MBL = 240 kN is available. (a) Calculate its SWL if used as a powered crane hoist rope. (b) Calculate its SWL if the same physical rope were instead installed, unmodified, as a personnel lift rope. (c) Explain why the same rope gives two very different SWL values in (a) and (b).Show worked solution →
(a) Powered crane hoist (FoS = 5).
(b) Personnel lift (FoS = 12).
(c) Explanation. The rope's MBL of 240 kN does not change; only the required factor of safety changes with the consequence of failure. A personnel lift carries a much higher factor of safety to allow for shock loading, dynamic effects and wear between inspections, because a failure risks human life directly, whereas a goods hoist failure mainly risks equipment or cargo. Dividing the same MBL by a larger FoS gives a smaller allowable SWL.
Marking criteria: 1 mark for correct SWL in (a), 1 mark for correct SWL in (b), 1 mark for stating the MBL is unchanged, 1 mark for identifying that FoS reflects consequence of failure/risk to life, 1 mark for linking this correctly to why (b) is smaller than (a).
core4 marksA mobile crane's hook block uses a 2-fall reeving system with 14 mm 6 x 19 IWRC rope, MBL = 110 kN. AS1418 requires FoS = 5 for a powered crane hoist. (a) Calculate the SWL of a single rope segment. (b) Calculate the maximum load that may be lifted on the hook.Show worked solution →
(a) SWL of one rope segment.
(b) Maximum hook load. With 2-fall reeving, two rope segments support the load, each carrying half the total hook load, so the hook load is twice the SWL of a single segment:
Marking criteria: 1 mark for correct single-segment SWL, 1 mark for recognising 2-fall reeving doubles the supported load, 1 mark for correct multiplication, 1 mark for the correct final answer with units (44 kN).
core5 marksA routine inspection of a tower crane hoist rope of nominal diameter 20 mm finds 4 broken outer wires within one lay length, all located in a single strand, and a measured diameter of 18.6 mm at the worn section. Using AS2759 discard criteria, determine, with reasons, whether the rope must be retired from service.Show worked solution →
Broken wire check. AS2759 retires a rope when three or more broken wires occur within a single strand in one lay length. This rope has 4 broken wires in one strand in one lay length, which exceeds the 3-wire threshold, so retirement is triggered on this criterion alone.
Diameter check.
A reduction of 7 percent or more from nominal diameter is also a retirement trigger, so this rope fails the diameter criterion as well.
Conclusion. The rope must be retired immediately; it fails two independent AS2759 criteria (broken wires in a single strand, and diameter reduction), either of which alone would be sufficient grounds for discard.
Marking criteria: 1 mark for correctly applying the broken-wire-in-one-strand threshold, 1 mark for the correct percentage diameter reduction calculation, 1 mark for correctly comparing this to the 7 percent threshold, 1 mark for a clear discard conclusion, 1 mark for correctly identifying that the rope fails on two separate criteria.
exam6 marksAssess the suitability of Lang's lay wire rope, compared with ordinary (regular) lay, for the main hoist rope of a ship-to-shore container crane at Port Botany, where the rope repeatedly runs over boom-tip sheaves under high cyclic load.Show worked solution →
This is a 6-mark ASSESS: markers reward a supported judgement contrasting both lay types against the specific operating conditions, not just a description of each.
- Key contrast
- Lang's lay wires run at an angle across the rope surface (wires and strands laid the same direction), which spreads wear over a longer wire length as the rope bends repeatedly over a sheave, giving it materially better fatigue life under the high-cycle bending Port Botany's container crane experiences during continuous ship loading. Ordinary lay, with wires running near-parallel to the rope axis (wires and strands laid in opposite directions), resists crushing and untwisting better and is self-restraining, but its fatigue life under repeated sheave bending is lower.
- The key risk with Lang's lay
- Because both the wires and strands twist the same way, a Lang's lay rope has a strong tendency to rotate and untwist under load unless both ends are restrained against rotation (for example, by a swivel-free termination or a rotation-resistant multi-layer construction). An unrestrained Lang's lay rope on a single-part hoist can spin, causing load instability and accelerated wear.
- Judgement for this application
- A ship-to-shore container crane hoist is a high-cycle, sheave-intensive application where fatigue life is the dominant failure mode, favouring Lang's lay. Because modern container crane hoist systems are multi-part (reeved through several sheaves with the drum end fixed against free rotation), the untwisting risk can be adequately controlled by design, so Lang's lay (or a rotation-resistant equivalent) is the more suitable choice overall, provided the termination and reeving genuinely restrain rotation; if the crane instead used a simple single-part, rotation-free reeving, ordinary lay would be the safer default.
Marker's note: top-band answers (1) explicitly contrast the wire angle and its effect on fatigue life versus untwisting for both lay types, (2) apply this specifically to the high-cycle sheave bending of a container crane rather than describing lay types generically, (3) identify the rotation-restraint condition needed to safely use Lang's lay, and (4) close with an explicit, justified judgement rather than a neutral summary.
exam6 marksA personnel lift cage at an underground mine is supported by 4 parallel wire rope falls, each rope having MBL = 180 kN. AS1418 requires FoS = 12 for personnel lifts. (a) Calculate the SWL of a single rope fall. (b) Calculate the total rated capacity of the cage, in kN and in tonnes. (c) During an emergency stop, dynamic loading raises the actual rope tension to 1.8 times the static tension. For a fully loaded cage of total mass 2000 kg (passengers plus cage), calculate the tension per rope fall during the emergency stop and comment on how this compares with a single rope's MBL.Show worked solution →
(a) SWL of a single rope fall.
(b) Total rated capacity.
(c) Emergency stop tension per rope.
Static weight of the loaded cage:
Shared equally across 4 falls, static tension per rope:
Dynamic tension during the emergency stop:
Comment. Even under the 1.8 times dynamic spike, the tension in a single rope (8.83 kN) remains far below that rope's MBL of 180 kN, a realised safety margin of roughly 20 times. This shows the FoS of 12 applied at the design stage comfortably absorbs this emergency-stop event with margin to spare, confirming the cage remains safe even during the dynamic loading of a sudden stop.
Marking criteria: 1 mark for correct SWL per fall, 1 mark for correct total SWL, 1 mark for correct conversion to tonnes, 1 mark for correct static weight and per-rope static tension, 1 mark for correctly applying the 1.8 dynamic factor, 1 mark for a correct comparison/comment relating the dynamic tension to the rope's MBL.