Winch Drum Line Pull by Layer
Derated line pull and increased speed of a winch drum at a given rope layer.
Example
You enter
- Bare-drum rated pull (lb) 10000
- Bare drum diameter (in) 10
- Wire rope diameter (in) 0.5
- Drum barrel width (in) 12
- Target layer (1 = bare) 4
You get
- Mean diameter at layer 13.5 in
- Pull at layer 7407
- Wraps per layer 24
Details, formula, and sources
The derated line pull, mean diameter, and increased speed of a winch drum at a given rope layer (Pn = rated x drum / (drum + (2n-1) x rope)). The rated pull is a bare-drum first-wrap figure that fades 30-40% on the outer layers -- plan the pull for the layer you finish on, not the nameplate.
Dn = drum_dia + (2n - 1) x rope_dia; Pn = rated_pull x drum_dia / Dn; Vn = drum_speed x Dn / drum_dia; wraps_per_layer = floor(barrel_width / rope_dia).
Wire-rope drum mechanics / SAE winch rating convention, by name.
Drum line-pull mechanics are a public engineering result; the winch manufacturer's layer ratings govern.
Estimate. Head rigger and manufacturer working-load-limit charts govern. Inspect every piece of hardware before the show.
Field names used by the API: rated_pull_lb, drum_dia_in, rope_dia_in, barrel_width_in, target_layer, mean_dia_in, pull_at_layer_lb, wraps_per_layer
- Mean-diameter model Dn = drum + (2n-1) x rope_dia; pull inversely proportional to mean diameterdrum mechanics
- Layer ratings govern the winch manufacturer's published per-layer ratings are authoritativeSAE winch rating