| Scenario | Load | Legs / Loaded legs | Angle method | Angle (from horizontal) | Dynamic factor | Estimated leg tension (each) |
|---|---|---|---|---|---|---|
| Two-leg bridle, typical pick | 20 kN | 2 / 2 | Direct | 60° | 1.00 | 11.55 kN |
| Four-leg bridle, conservative | 40 kN | 4 / 2 | Direct | 45° | 1.10 | 31.11 kN |
| Geometry-based check | 30 kN | 2 / 2 | Height 3, Span 6 | 45° | 1.00 | 21.21 kN |
Examples are illustrative. Your rigging and load geometry may require engineering review.
The calculator converts the entered load to a common force basis, then applies:
W = lifted load, A = allowance for rigging gear, D = dynamic factor.
Angle from horizontal is computed by geometry when selected:
θ is the sling angle from horizontal. Angle from vertical is α = 90° − θ.
For n loaded legs sharing the lift:
Components per leg:
Angle factor is 1/sin(θ). Lower angles amplify tension and horizontal force.
- Enter the load and select the correct unit.
- Add an allowance if the hook, spreader, or rigging adds weight.
- Set a dynamic factor for movement or potential shock loading.
- Select legs and choose a load-sharing assumption.
- Choose an angle method and provide angle or geometry values.
- Click Calculate and review tension and warnings.
- Keep sling angles as large as practical to reduce tension.
- For 3- or 4-leg bridles, uneven load sharing is common.
- Confirm hardware ratings, sling type, and inspection status.
- Consider edge protection, center of gravity, and lift control.
Angle selection and lift capacity planning
Sling angle directly changes leg tension through the angle factor, which is calculated as 1/sin(θ) when θ is measured from horizontal. At 60°, the factor is about 1.155, but at 30° it doubles to 2.000. This means the same load can demand nearly twice the sling capacity per leg as angles get shallow, even before dynamic effects.
Leg count versus loaded legs assumptions
Bridle assemblies with three or four legs often do not share load equally due to center-of-gravity offsets, stiffness differences, or connection geometry. The calculator allows equal sharing, conservative two-leg sharing, or a custom loaded-leg count. Using conservative sharing often yields a safer planning check for real-world conditions where one leg can go slack.
Dynamic factor and rigging allowance controls
Field lifts rarely behave like static textbook cases. A dynamic factor accounts for acceleration, hoist starts and stops, wind, and minor impacts. A small allowance can also be added for hooks, shackles, spreaders, and lifting beams. Together, these inputs build a practical total load used for tension calculations and help prevent underestimating demand.
Horizontal forces and stability considerations
As the sling angle decreases, the horizontal component rises because H = T·cos(θ). High horizontal force can spread pick points, increase side loading on hardware, and amplify rotation risk. Reviewing both per-leg horizontal force and total horizontal force supports better decisions on using spreader beams, tag lines, and controlled lift paths.
Reporting, checking, and sharing lift notes
The results panel summarizes total load used, leg tension, and risk band based on angle from horizontal. Geometry mode computes θ from hook height and span using a symmetric assumption, which is useful for quick feasibility checks. Exporting CSV or PDF supports toolbox talks, lift plans, and consistent documentation across crews and shifts.
What angle should I target for most routine picks?
Many crews aim to keep sling angles at or above 45° from horizontal when feasible, because tension amplification remains moderate. Always confirm with site rules, equipment ratings, and the lift supervisor’s requirements.
Why does a four-leg bridle sometimes act like two legs?
Uneven geometry and center-of-gravity shifts can unload one or two legs, leaving only two legs carrying most of the load. Conservative planning accounts for this common field behavior and avoids overestimating capacity.
Can I use kilograms or tonnes in the inputs?
Yes. When you choose kg or t, the calculator converts weight to force using standard gravity, then computes tensions. For strict engineering, use your project’s specified gravity or the unit system required by procedures.
How does geometry mode calculate the sling angle?
It assumes a symmetrical bridle and uses θ = arctan(Height ÷ (Span/2)). Height is the hook rise above pick points, and span is the distance between pick points. Keep length units consistent.
Do the results include sling weight and hardware mass?
Only if you enter a rigging allowance. Add the estimated weight of hooks, shackles, spreaders, and slings as an allowance so the computed total load reflects what the crane and rigging actually support.
Is this output a substitute for a formal lift plan?
No. It is a planning aid to understand angle-driven tension changes. For critical lifts, follow your lift planning process, verify rated capacities, inspect gear, and use qualified personnel to approve the final rigging method.