Power Screw Calculator — Raise/Lower Torque, Efficiency & Self-Locking (Shigley §8-2)

The governing METHOD is the power-screw friction-circle model of Shigley's Mechanical Engineering Design (Budynas & Nisbett, 11th ed., §8-2) — raise and lower torque, thread efficiency, the self-locking criterion, bearing pressure and the root stresses all come from there. ASME B1.5 (Acme), DIN 103 / ISO 2901 (metric trapezoidal) and ANSI B1.9 (buttress) are DIMENSIONAL thread standards: they supply the flank angle α and the thread dimensions, and contain no torque or efficiency model of their own. The modified friction coefficient μ′ = μ / cos(α) is applied to every flanked form; square threads use α = 0. The full method, with the substituted values, is printed in the generated PDF report.

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 load cannot back-drive the screw — no brake or holding device is needed. When λ > φ′ the screw overhauls and will back-drive under load unless a separate braking torque is applied. The calculator evaluates this in real time and displays a SELF-LOCKING / BACK-DRIVES badge.

Four power-thread forms: Acme (ASME B1.5, 29° included, α = 14.5° — the most common inch form), metric trapezoidal Tr (DIN 103 / ISO 2901, 30° included, α = 15° — the metric equivalent), square (α = 0°, the highest efficiency, no flank force component) and buttress (ANSI B1.9, asymmetric with a 7° load flank and a 45° back flank, for unidirectional load; the DIN 513 variant uses a 3° load flank instead). Select the thread type in the header; the flank half-angle and the modified friction coefficient μ′ = μ / cos(α) follow automatically. Only the LOAD flank angle enters μ′, because that is the flank carrying W.

Thread efficiency η counts only the work done at the thread helix — it excludes collar (thrust-bearing) losses. Total or handle-referred efficiency is lower because it includes the collar torque μ_c·W·(d_c/2) in the denominator. On the tool's default screw the thread efficiency is 36.0 % while the overall figure is 20.3 %: sizing a motor from the thread figure would undersize it by about 44 %. The efficiency map plots thread efficiency; the mechanical-advantage panel and the hero sub-line both show the overall handle-referred figure (MA_actual / MA_ideal × 100), and the reported shaft power P is always computed from the TOTAL torque.

No — and for a long unsupported screw in compression, column buckling is frequently the failure mode that governs, well before the torque or thread limits are reached. This tool computes the axial and torsional stresses at the root diameter and combines them by Von Mises; it has no slenderness or end-condition input. Take the root diameter d_r it reports, together with your axial load and the unsupported length, to the Euler/Johnson buckling calculator. The tool links to it directly, and the assumptions section of the PDF report states the omission.

Because the safe bearing pressure for a sliding screw and nut is a function of rubbing speed, not of the material pair alone. Shigley's Table 8-4 gives roughly 17–24 MPa for a steel screw on a bronze nut at low speed, about 11–17 MPa above 10 ft/min, about 5.5–9.7 MPa at 20–40 ft/min and far less beyond that. The calculator computes the rubbing velocity at the mean diameter from your speed and de-rates the permissible pressure accordingly, and it also compares against the permissible pressure you enter for your own material pair in the thread-strength panel — a cast-iron or PTFE-lined nut is governed by a lower figure than the built-in bronze one.

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.