KEVOS® MAKEOS · Mechanics
Capstan and Bollard Rope Turns Check (Holding Force, Turns Needed, Rope Load)
How many turns of rope around a bollard, capstan or drum let a person hold this load, and is the rope strong enough?
Check of rope turns on a bollard, capstan, winch drum or post: the ratio of tight to slack tension from the friction coefficient and wrap angle (as the belt friction tool, capstan equation), the holding force needed with the turns used, the turns needed for the holding force a person can apply, and the rope working load from its breaking strength and safety factor, against the load.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Load (tight side tension) | N | Tension on the load side of the rope. |
| Holding force a person can apply | N | Force the person tending the rope can safely hold. |
| Friction coefficient of rope on the post | Friction coefficient between the rope and the post or drum (wet, glazed or rope-on-rope contact is lower). | |
| Turns used | turns | Complete turns of rope around the post (one turn = 360° of wrap). |
| Rope | ||
| Rope minimum breaking load | kN | Minimum breaking load of the rope. |
| Safety factor on the rope | Safety factor applied to the breaking load for the working load. | |
Method and formulas
- T₁ ÷ T₂ = e^{μθ} with θ = 2π × turns (as the belt friction tool).
- Holding force = T₁ ÷ e^{μθ}; turns needed = ln (T₁ ÷ T₂) ÷ (2π μ).
- Working load = breaking load ÷ safety factor.
- Example: 3 turns give a ratio of 111.32; hold 179.67 N; at least 2.6736 turns are needed.
Limitations
- Rope fully bedded on a round post; rope stiffness and crossing turns change the friction.
- Static holding; surging a rope round a capstan heats and wears it.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Find the turns needed to snub a load on a post
- Check a capstan winch line pull
- Lower a load by hand round a bollard
Who it is for
- Riggers
- Arborists
- Marine and rescue crews
What you get
- Tension ratio for the turns used
- Holding force needed and turns needed
- Rope working load
Interpreting the result
- Each extra turn multiplies the holding power by the same large factor, which is why a few turns hold a heavy load.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 165–168, 2473–2490. Topic reference; no source tables or recommended settings are reproduced.
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Open the Capstan and Bollard Rope Turns Check (Holding Force, Turns Needed, Rope Load) calculator
