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 7777.8
- Wraps per layer 24
Checked against published worked examples from: Wire-rope drum mechanics / SAE winch rating convention.
Details, formula, and sources
ScopeThe derated line pull, mean diameter, and increased speed of a winch drum at a given rope layer (Pn = rated x (drum + rope) / (drum + (2n-1) x rope), the rating being the first-layer figure at the first wrap's centerline). 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.
Formula or tableDn = drum_dia + (2n - 1) x rope_dia; Pn = rated_pull x (drum_dia + rope_dia) / Dn (the rating is the first-layer figure at the first wrap's centerline); Vn = first-layer speed x Dn / (drum_dia + rope_dia); wraps_per_layer = floor(barrel_width / rope_dia).
Source and editionWire-rope drum mechanics / SAE winch rating convention, by name.
Free accessDrum line-pull mechanics are a public engineering result; the winch manufacturer's layer ratings govern.
What governsEstimate. Head rigger and manufacturer working-load-limit charts govern. Inspect every piece of hardware before the show.
API field namesrated_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 diameter, referenced to the first layer at drum + rope_diadrum mechanics
- Layer ratings govern the winch manufacturer's published per-layer ratings are authoritativeSAE winch rating