ASME B1.5 Acme Thread Calculator — Power Screw Torque, Efficiency & Self-Locking
ASME B1.5 — Acme Screw Threads
ASME B1.5 is the American National Standard that defines the geometry and dimensional requirements for Acme screw threads — the trapezoidal 29° included-angle (14.5° half-angle) thread form used in power-transmission lead screws, screw jacks, vises, presses and linear actuators. It specifies the basic thread profile, major and minor diameters, lead, pitch, tolerances and thread classes (2G general-purpose, 3G and 4G close-tolerance, and 5G centralising) that govern how Acme threads are manufactured and inspected.
The standard is paired with the Shigley lead-screw friction-circle model (Budynas & Nisbett, §8-2) to compute the raising and lowering torques, mechanical efficiency, and the self-locking condition — the analysis that determines whether a screw jack holds a load without a brake. MechanixCalc implements this model directly in the browser for Acme, square and buttress thread forms, computing the complete torque budget (thread torque, collar torque), efficiency map, Von Mises stress at the root, thread contact pressure, and the Archard wear life, with a shareable PDF report showing the worked method.
Calculators that implement ASME B1.5
What ASME B1.5 covers
- Acme thread form geometry — 29° included angle (14.5° flank half-angle α), basic profile, major/minor/pitch diameters, thread depth and flat widths
- Thread classes — 2G (general-purpose), 3G and 4G (close tolerance), 5G (centralising); allowances and tolerances for each class
- Lead and pitch conventions for single- and multiple-start Acme threads
- Modified friction coefficient μ′ = μ / cos α — the correction that accounts for the radial force component on the inclined 14.5° flank, elevating the effective friction relative to a square thread
- Raise and lower torque model — friction-circle analysis at the thread mean radius, plus collar (thrust-bearing) torque in series
- Self-locking condition: tan(λ) < μ′, where λ is the lead angle — the criterion that determines whether a back-drive brake is required
Governing formulas
T_raise = W · r_m · (l + π·μ′·d_m) / (π·d_m − μ′·l) + μ_c · W · (d_c / 2)where W = axial load (N); r_m = d_m / 2 = mean thread radius (mm); d_m = mean thread diameter = d − h (mm); l = lead (mm/rev); μ′ = μ / cos α = modified thread friction coefficient (α = 14.5° for Acme); d_c = collar (thrust-bearing) mean diameter (mm); μ_c = collar friction coefficient
Self-locking: tan(λ) < μ′ where λ = arctan(l / (π·d_m))
η = (W · l) / (2·π · T_raise_thread) × 100 %where λ = lead angle; μ′ = modified friction coefficient; η = thread efficiency (excludes collar losses); T_raise_thread = thread torque component only
σ_vm = √(σ_c² + 3·τ²)where σ_c = W / (π·d_r² / 4) = compressive stress (MPa); τ = 16·T_raise_thread / (π·d_r³) = torsional shear stress (MPa); d_r = d − 2h = minor (root) diameter (mm)
Frequently asked questions
What is ASME B1.5 used for?
ASME B1.5 specifies the geometry and tolerances for Acme screw threads — the 29° trapezoidal thread form used in power-transmission lead screws, screw jacks, vises, presses and linear actuators. It defines the thread profile, major and minor diameters, lead, pitch, and thread classes (2G–5G) that govern manufacture and inspection. Combined with the Shigley friction-circle model it also underpins the torque, efficiency and self-locking calculations used to size and verify a lead-screw drive.
What is the flank angle of an Acme thread under ASME B1.5?
An Acme thread has a 29° included angle, which means a 14.5° half-angle (flank angle α = 14.5°) on each side of the thread form. This inclined flank produces a radial force component that raises the effective friction to μ′ = μ / cos(14.5°), which is approximately 3.3 % higher than the flat-flank (square-thread) value at the same nominal friction coefficient. The modified friction coefficient must be used in the torque and self-locking equations to obtain the correct answer.
How is the self-locking condition determined for an Acme power screw?
A power screw is self-locking when the lead angle λ is less than the friction angle φ′ = arctan(μ′), which is equivalent to tan(λ) < μ′. When this condition holds, the axial load cannot back-drive the screw — no brake or holding device is needed. If the lead is coarse enough that λ > φ′, the screw will overhaul (back-drive) under load and a separate braking torque must be provided. The MechanixCalc power screw calculator evaluates this condition in real time and displays a SELF-LOCKING or BACK-DRIVES badge.
How does ASME B1.5 relate to DIN 103 and ISO 2901?
All three standards govern trapezoidal power-thread forms, but with different flank angles: ASME B1.5 Acme uses a 29° included angle (14.5° half-angle), while DIN 103 / ISO 2901 metric trapezoidal threads use a 30° included angle (15° half-angle). The torque and efficiency formulas are structurally identical — the only numerical difference is the flank angle used to compute μ′ = μ / cos α. The MechanixCalc power screw calculator supports both standards and selects the correct α automatically based on the thread type chosen.
Is the ASME B1.5 power screw calculator free?
You can run a full calculation during a free 30-minute preview with no sign-up required, and a free 14-day account trial (no credit card) unlocks every calculator on the platform. The branded PDF engineering report and saved / shareable calculations are part of a paid plan.
Related standards
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