Mechanical Design

Power Screw Calculator

Axial force, efficiency, and self-locking analysis for ACME, square, and large-thread power screws — jacks, clamps, presses, and lead screws.

Inputs

Thread geometry

5 TPI
deg
Multi-start: lead = starts × pitch

Load / torque

🔒
Self-locking

Results

Axial force (F)

—

lbs

Applied torque

—

in·lb

Mechanical efficiency (η)

—

%

Lead angle (λ)

—

degrees

Self-lock angle (φ)

—

degrees

Self-locks when λ < φ

Back-drive torque

—

in·lb

Standard ACME threads — lead angle & self-lock (µ = current input)

Thread TPI Lead (in) dm (in) Lead angle λ φ (friction angle) Self-locking? η (raising)

Power Screw Equations (Shigley / Machinery's Handbook)

Raising: T_r = F·dm/2 · (l + π·µ'·dm) / (π·dm − µ'·l) Lowering: T_l = F·dm/2 · (π·µ'·dm − l) / (π·dm + µ'·l) µ' = µ / cos(α)   [modified friction for V/ACME thread] dm = D − p/2   [mean thread diameter] l = starts × p   [lead] λ = arctan(l / π·dm)   [lead angle] φ = arctan(µ')   [friction angle] Self-locking: λ < φ   (i.e. µ > tan λ · cos α) η = F·l / (2π·T_r)   [raising efficiency] Back-drive torque = T_l (positive = requires torque to lower; negative = self-locking)
α = thread half-angle (0° square, 14.5° ACME, 30° V-thread) dm = mean thread diameter l = lead (= pitch × starts) µ' = modified friction coefficient λ = lead angle φ = friction angle