3D Model — Orbit & Inspect
Scenario — Reeved Sling Timber Pole Lift
The Task
A timber pole rigged with 3-tonne reeved slings. Students inspect the rigging configuration and work through the weight calculation to verify the slings are adequate.
Sling Type
Reeved round slings — the sling passes around the load and back through a shackle or ring, effectively doubling the lifting capacity of each leg.
Sling Angles
Observe the included angle between sling legs. As the angle increases, the load on each leg increases. Never exceed 120°.
Attachment Points
Check the attachment positions along the pole relative to the centre of gravity. Incorrect placement causes the load to tilt during lift.
Balance Point
The sling attachment points must be positioned so the load hangs level. For uniform timber, this is near the geometric centre — but knots, moisture, and taper shift the CG.
Timber Weight Calculation
Weight Formula
Volume in m³ • Density in kg/m³ • Result in kg
| Timber Type | Density (kg/m³) | Notes |
|---|---|---|
| Treated Pine | 600 – 800 | Common structural, CCA treated |
| Hardwood (seasoned) | 900 – 1,100 | Ironbark, spotted gum, etc. |
| Green (unseasoned) | +30–50% | Heavier than seasoned — contains moisture |
Critical Warning
- Timber is a natural material — density varies between species, within the same species, and even along the same log
- Wet timber can be 50% heavier than dry timber of the same species
- Always check for hidden metal — old bolts, nails, brackets add unexpected weight
- When in doubt, always overestimate the weight
Cylinder Volume Calculation
Volume of a Cylinder
r = radius in metres • L = length in metres • Result in m³
Worked Example
300 mm diameter × 6 m pole:
r = 0.15 m
V = π × 0.15² × 6 = 0.424 m³
At 800 kg/m³ (treated pine):
Weight = 0.424 × 800 = 339 kg
Calculation Tips
- Measure diameter at the thickest point for safety margin
- For tapered poles, use the average of the two end diameters
- Always round UP to the nearest safe working figure
- Add 10–15% safety margin for unknowns (moisture, knots, hidden metal)
Reeved Sling Factor
When a sling is reeved through a shackle or around the load and back, the effective WLL increases because the load is shared across multiple parts of the sling.
| Configuration | Reeve Factor | Effective WLL |
|---|---|---|
| Single part (straight) | 1.0 | Sling WLL × 1.0 |
| Reeved (doubled back, parallel legs) | 2.0 | Sling WLL × 2.0 |
Sling Angle Reduces the Doubled Factor
The 2.0 factor applies only when both parts of the reeved sling hang parallel (0° included angle). As the legs spread, capacity is reduced by the AS 3775 sling-angle factor:
| Included angle | Effective factor |
|---|---|
| 0° (parallel) | 2.00 |
| 60° | 1.73 |
| 90° | 1.41 |
| 120° (maximum permitted) | 1.00 |
Source: AS 3775 Chain Slings — sling-angle factors per wwwroot/knowledge-base/verified-safety-data.json#slingAngleFactors. Included angles greater than 120° are prohibited.
Critical Rules
- The sling must run freely through the reeving point — if it binds, the load is not shared equally
- Ensure the reeving point (shackle pin, hook) is rated for the full load
- Both parts of the reeved sling must be the same type and WLL
- Inspect the sling at the reeving point — this is where abrasion damage occurs
Standards & References
- AS 4991 — Lifting equipment: General requirements for the design, manufacture, and testing of lifting equipment
- NSW Dogging & Rigging Guide — Safe Work NSW guide for dogging and rigging operations, including sling angle tables and WLL calculations
- AS 1353.1 — Flat synthetic-webbing slings (relevant to round sling construction)
- AS 4497.1 — Round slings — Synthetic fibre
- WHS Regulation 2017 (NSW) — Chapter 5, Part 5.1 — General workplace management: Plant and structures
Before Every Lift
- Calculate the load weight — never guess
- Select slings with adequate WLL including angle derating
- Inspect all slings and hardware before use
- Confirm attachment points relative to centre of gravity
- Conduct a trial lift to check balance before full lift