How it works
A rope running over a block or through a rigging point pulls on it along both legs. The two pulls are vectors, so the force on the point is their vector sum. When the legs are close together the pulls add almost directly; when the rope runs nearly straight through, they largely cancel.
With equal tensions T and an angle φ between the legs, the point load is 2 T cos(φ/2): 1.41 T at 90°, exactly T at 120°, and nearly 2 T when the rope doubles back. A dynamic factor raises T for shock loading; friction at the block lowers the holding-side tension.
Worked example
A 500 lb piece lowered through a block with the rope turning 90° (legs 90° apart), static loading:
- Load-side tension = 500 lb × 1 = 500 lb
- Holding-side tension = 500 × 100% = 500 lb
- Force = √(500² + 500² + 2 × 500 × 500 × cos 90°) = 707 lb
| Input | Value |
|---|---|
| Load on the rope | 500 lbf |
| Dynamic factor | 1 × |
| Holding-side tension as % of load-side | 100 % |
| Angle between the two rope legs | 90 ° |
| Result | Value |
|---|---|
| Force on the rigging point | 707 lbf |
| Multiple of the load | 1.41 × |
| Load-side tension | 500 lbf |
| Holding-side tension | 500 lbf |
Assumptions and limits
- The block and rope are ideal: friction is set only by the holding-side percentage you enter.
- The dynamic factor is your input. The actual shock load of a falling piece depends on the fall distance and rope stretch; see the shock-load calculator.
- Only the rope forces on the point are calculated; the strength of the tree, sling, and hardware is not.
Common questions
Why is the point load bigger than the load?
A rope running through a block pulls on it twice, once on each leg. Unless the rope runs straight through, the two pulls add, so the point carries more than the load weighs.
What angle gives exactly the load?
A 120° angle between equal legs gives a point load equal to one leg’s tension. Wider than that and the load drops; narrower and it rises toward twice the tension.
How do I account for shock loading?
Enter a dynamic factor above 1, or use the rigging shock load calculator to estimate the peak force first.
Does friction help?
A friction device at the point lowers the holding-side tension, which lowers the point load. Reduce the holding-side percentage only if you know the friction.
Sources
- Vector addition of forces (resultant of two concurrent forces), from standard statics.
- Special cases 2T at 0° between legs, 1.41T at 90°, and T at 120° follow from the same equation.
Updated 2026-09-30