Enter load and system details
Use the same basis for your load inputs. The angle is measured from the horizontal.
Formula used
Per-leg WFL = (Load × Dynamic Factor) ÷ (Legs × sin(Angle) × Efficiency)
Required MBL = Per-leg WFL × Safety Factor
Efficiency is written as a decimal in the calculation. For example, 90% becomes 0.90. The formula assumes equal loading between supporting legs. Uneven loading requires a more conservative engineering approach.
How to use this calculator
- Enter the real load and select its unit.
- Choose a dynamic factor that matches your lift conditions.
- Enter the legs that truly support the load.
- Measure each sling angle from the horizontal.
- Apply verified system efficiency or derating.
- Enter the required safety factor for breaking-strength checks.
- Review the per-component working limit before selecting equipment.
- Use the optional strength field to check available capacity.
Example load data
| Input | Example value | Purpose |
|---|---|---|
| Applied load | 12 kN | Base lifted force |
| Dynamic factor | 1.10 | Allows for controlled movement |
| Supporting legs | 2 | Balanced two-leg connection |
| Sling angle | 60 degrees | Measured from horizontal |
| System efficiency | 90% | Applies a conservative derating |
| Safety factor | 5 | Derives minimum breaking strength |
This example produces a required per-leg working force limit of about 8.47 kN. The corresponding minimum breaking strength is about 42.34 kN.
Working Force Limits in Lifting Systems
A working force limit is the greatest service load a component should carry. It protects people, equipment, and structures. It is not the same as a breaking strength. Breaking strength describes failure testing. Working force limit includes a safety margin for real conditions.
Lifted loads rarely stay perfectly still. Starting, stopping, and swinging can increase force. Wind and poor balance can create extra stress. Slings also change force direction. A shallow sling angle increases tension in every supporting leg. This calculator accounts for those effects.
The first input is the applied load. Use the actual lifted mass or force. Choose a matching unit before calculation. A mass entered as kilograms-force already represents weight under normal gravity. Newton and kilonewton values represent force directly. Convert values carefully when records use different units.
The dynamic factor allows for motion. A value of one represents a steady load. A larger value represents acceleration, impact, or uncertain movement. The calculator multiplies the applied load by this factor. The result becomes the adjusted design load. Select a factor supported by your site procedures.
Supporting legs share the load only when geometry is balanced. Equal load sharing is an assumption. Uneven lengths, poor connections, and off-center loads can overload one leg. Treat the number of legs cautiously. Use the number that reliably carries the load. Follow rigging guidance for complex lifts.
Sling angle is measured from the horizontal. A vertical leg has an angle of ninety degrees. Lower angles produce a smaller sine value. Therefore, each leg must resist more tension. Never assume two slings always halve the load. The angle may make each leg carry far more than half.
Efficiency represents losses from hardware, connections, and system behavior. One hundred percent means no derating. Lower values increase the required rating. Use manufacturer data where available. Do not invent efficiency values for safety-critical equipment. Conservative assumptions are safer than optimistic estimates.
The calculator reports adjusted total load and per-leg tension. The per-leg result is the required working force limit. It also multiplies that value by the safety factor. This gives a minimum breaking strength target. Compare both numbers with rated labels and inspection records.
An optional breaking-strength input estimates allowable applied load. This feature reverses the process. It first divides breaking strength by the safety factor. Then it applies geometry, efficiency, and dynamic adjustments. The estimated allowable load remains theoretical. It never replaces an engineered lift plan.
Inspect every component before use. Check slings, hooks, shackles, anchors, and connectors. Remove damaged equipment from service. Confirm the load center and travel path. Keep people away from suspended loads. Stop work when conditions differ from the plan.
This tool supports screening and learning. It does not certify equipment or approve a lift. Local regulations and manufacturer instructions always control. Seek a competent person for high-risk, personnel-lifting, or unusual operations. Good calculations help. Careful supervision makes lifting operations safer.
Frequently asked questions
1. What does working force limit mean?
Working force limit is the maximum service force a component should carry under stated conditions. It is a rated operating limit. It differs from breaking strength, which describes the force that may cause failure during a controlled test.
2. Is working force limit the same as breaking strength?
No. Breaking strength is normally higher. A safety factor separates the two values. This calculator uses the safety factor to estimate the minimum breaking strength needed after calculating the required working limit for each supporting component.
3. Why does a shallow sling angle increase force?
A shallow angle provides less vertical support from each sling leg. The same lifted load then creates more tension in each leg. As the angle approaches horizontal, tension rises rapidly and can exceed rated capacity.
4. Which sling angle should I enter?
Enter the angle between one supporting leg and the horizontal plane. A vertical leg is 90 degrees. Do not enter the included angle between two legs unless you first convert it to the angle from horizontal.
5. What dynamic factor should I use?
Use 1.00 for a carefully controlled static condition. Use a higher factor when acceleration, stopping, impact, vibration, or uncertain movement is expected. Site rules, manufacturer guidance, and a competent lift planner should determine the chosen value.
6. Does this calculator assume equal load sharing?
Yes. It assumes each entered supporting leg shares load equally. Real systems can be uneven because of leg length, connection geometry, load center, and hardware. Use conservative engineering review when equal sharing cannot be assured.
7. What should system efficiency include?
System efficiency may include verified losses or derating from hardware, connections, reeving, and other system effects. Use documented values when possible. Do not use an optimistic percentage simply to make a selected component appear adequate.
8. Can I enter kilograms instead of force?
Use kilograms-force when entering weight as a force approximation. Ordinary kilograms describe mass, not force. The calculator labels kgf separately to reduce confusion. For high-accuracy or regulated work, use force units and approved procedures.
9. What does the optional breaking-strength field do?
It calculates the available working limit from the entered breaking strength and safety factor. It also estimates the maximum applied load and the component utilization. A result above 100% means the entered rating needs review.
10. Can this calculator approve a real lift?
No. It is a planning and screening tool. It cannot inspect equipment, assess site conditions, or verify regulations. A competent person must review critical lifts, personnel lifts, unusual rigging, and any condition outside the planned assumptions.
11. What should I check before lifting?
Confirm equipment ratings, inspections, connections, angle, load center, travel path, communication method, and exclusion zone. Verify that every component has suitable capacity. Stop and reassess whenever the lift changes from the approved plan.