KEVOS® MAKEOS · Mechanics
Rope Winch Drum and Capstan (Wraps, Holding Pull, Torque, Power)
How many turns of rope around this drum let a small tail pull hold the load, what torque and power does the drum need, and is the rope strong enough?
Check of a rope wrapped around a rotating drum or capstan: the tension ratio e^(μθ) for the turns of rope (as the belt friction tool with no groove), the largest load the tail pull can hold, the turns needed for the load, drum torque from the difference of tensions, drive power at the rope speed with the drive efficiency, and the rope safety factor against its breaking load.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Load on the rope | kN | Pull on the load side of the rope. |
| Tail (holding) pull | N | Pull applied by hand or a tailing device on the slack side. |
| Rope-to-drum friction coefficient | Friction coefficient between rope and drum (about 0.2–0.3 fibre rope on steel; lower when wet or oily). | |
| Turns of rope on the drum | Number of complete turns around the drum. | |
| Drum diameter (to rope centre) | mm | Diameter of the drum measured to the rope centre. |
| Rope speed | m/min | Speed of the rope. |
| Drive efficiency | % | Efficiency of the gearing and drive. |
| Rope minimum breaking load | kN | Minimum breaking load of the rope. |
| Rope safety factor required | Safety factor on the rope you require (from the lifting or rope standard). |
Method and formulas
- Wrap angle θ = 2π × turns; tension ratio = e^(μθ) (as the belt friction tool with no groove).
- Largest load held = tail pull × ratio; turns needed = ln(load ÷ tail) ÷ (2π μ).
- Drum torque = (load − tail) × D ÷ 2; power = load × speed ÷ efficiency.
- Rope safety factor = breaking load ÷ load.
- Example: 3 turns give a ratio of 111.32; 200 N holds up to 22.264 kN. The drum needs 480 N·m and 0.98039 kW.
Limitations
- Impending slip with constant friction; rope stiffness and drum fleet angle are not included.
- Rope riding up on itself reduces the effective turns.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Size a capstan or warping drum
- Find how many turns a rope needs on a bollard
- Check the drive of a rope haulage winch
Who it is for
- Design engineers
- Rigging supervisors
- Maintenance engineers
What you get
- Tension ratio and holding capacity
- Turns needed for the load
- Drum torque, drive power and rope safety factor
Interpreting the result
- Each extra turn multiplies the holding capacity: friction rises exponentially with wrap.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 165–168. Topic reference; no source tables or recommended settings are reproduced.
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