ASME B1.5 Acme Threads — Power Screw Torque, Efficiency & Self-Locking
ASME B1.5 — Acme Screw Threads
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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 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 static-friction test of whether a screw jack can back-drive under a steady load (vibration, shock or lubricant change can still back-drive a self-locking screw, so a suspended or safety-critical load needs a brake). MechanixCalc applies this model (it does not check thread classes or tolerances) to Acme (14.5°), metric trapezoidal (15°), square (0°) and buttress (7°) 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
Video walkthrough (3:25)
- 0:00 A screw jack: OK at 25 kN, FAIL at 55 kN
- 0:29 A thread is a ramp: torque and self-locking
- 0:45 Steps 1–4 — Thread, friction, load, limits
- 1:23 Step 5 — Checks and the screw yield
- 1:59 What if: faster, slipperier, heavier — and the fix
- 3:07 What is not checked, and the PDF report
Read the transcript
Here's a screw jack: a 40 mm Acme screw lifting 25 kN. The tool says it's OK, and raising it takes 157 N·m.
Raise the load to 55 kN, and the screw fails.
This is the MechanixCalc power screw calculator. It finds the torque and the efficiency, checks self-locking, and checks the screw and the nut against their limits. Let's go through it, step by step.
A thread is a ramp wrapped around a shaft. The load sits on the ramp, each turn lifts it by one lead, and friction decides how much torque it takes.
If the ramp is shallow, friction holds the load by itself. That's self-locking. Too steep, or too slippery, and the load drives the screw back down.
Step one is the thread: an Acme screw, 40 mm across, with an 8 mm lead and a 60 mm bronze nut. The tool adds the standard root clearance, so the screw's core is 31.5 mm.
Step two is friction: 0.12 on the thread and 0.10 on the collar. It's the biggest unknown here, so check both ends of the range.
Step three is the load: 25 kN, at 30 rpm.
Step four is the limits. The nut pressure and the thread shear have defaults, but the screw's yield is blank.
So the verdict is CHECK. Every limit is met, but the von Mises stress, 110.4 MPa, is only judged against a generic reference.
Enter the screw's yield, 355 MPa, and the verdict turns OK.
Raising it takes 157 N·m, the thread efficiency is 36%, and the screw is self-locking, with a margin of 1.75.
The nut pressure is 7.9 MPa, within the 9.6 allowed at this speed.
Now run it at 120 rpm. The allowed nut pressure falls to 1.6 MPa, so the verdict is CHECK.
The thread now rubs too fast for a bronze nut. Slow it down, or choose a ball screw.
Back at 30 rpm, lower the friction to 0.06. The efficiency climbs to 53%, but the screw is no longer self-locking. Under load, it would turn back down by itself.
Now raise the load to 55 kN.
The von Mises stress at the thread root is 242.9 MPa, above the 200 MPa reference, and the screw fails.
The fix is a bigger thread: a trapezoidal Tr 60 by 9, with a 90 mm nut.
At 55 kN, the stress is 129.6 MPa and the nut pressure 7.0 MPa. The verdict is OK, and it still self-locks.
What the tool does not check is listed too: column buckling, fatigue, shock loads, whirling at speed and the nut's own thread bending. So silence never reads as a pass.
Every input, every formula and every check goes into a PDF report. A report your reviewer can follow.
That's a power screw, checked. Try it on your own design, on MechanixCalc.com.
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 and tolerances — defined by ASME B1.5 (general-purpose and centralizing series); the MechanixCalc tool has no thread-class or tolerance input and uses the basic profile only
- Lead and pitch conventions for single- and multiple-start Acme threads
- NOT in ASME B1.5, but computed on its geometry: the modified friction coefficient μ′ = μ / cos α, which accounts for the radial force component on the inclined 14.5° flank and raises the effective friction relative to a square thread
- NOT in ASME B1.5: the raise/lower torque model (friction-circle analysis at the thread mean radius plus collar torque in series) is power-screw mechanics from Shigley §8-2 and Machinery's Handbook, applied to the B1.5 thread form
- NOT in ASME B1.5: the self-locking criterion tan(λ) < μ′ (λ = lead angle), which determines whether a back-drive brake is required, is likewise textbook power-screw mechanics. ASME B1.5 specifies the thread FORM — profile, diameters, classes and tolerances — and contains no friction model, torque equation or self-locking rule
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 = max( √(σ_b² + σ_c² + σ_b·σ_c + 3·τ_drive²) , √(σ_c² + 3·τ_body²) , √3·τ_drive ), σ_b = max( 12·F₁·h/(π·d_r·p²) , 6·F₁/(π·d_r·p) ), F₁ = 0.38·Wwhere σ_c = W / (π·d_r² / 4) = compressive stress (MPa); τ_body = 16·max(T_thread, T_collar) / (π·d_r³), τ_drive = 16·T_raise / (π·d_r³) = torsional shear stresses (MPa); σ_b = thread-root bending of the first thread, 0.38·W on one thread by the Shigley §8-2 method (more when fewer than three threads engage), p = thread pitch; d_r = d − 2h − 2·a_c = the screw's root (core) diameter (mm) — a_c the root clearance, 0.010 in per side for a general-purpose Acme at 10 tpi and coarser
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 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, a steady axial load does not back-drive the screw in static friction — but vibration, shock or lubricant change can back-drive a self-locking screw — fit a brake for a suspended or safety-critical load. The calculator prints the margin μ′ / tan(λ) with the verdict. 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 thread forms 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 included in the free 14-day trial and in every paid plan.
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