Engineering Calc Tools

Machine Design

Power Screw Torque Calculator

Torque to raise and lower a load, efficiency and self-locking for square and Acme power screws with collar friction.

Tool no.
ECT-M07
Rev
1.0
Updated
Oct 2026
Method
Shigley power screw equations
How to use this calculator

Enter the thread form, major diameter, pitch and number of starts, the load and the friction coefficients for the thread and the thrust collar.

Formulas

Lead and mean diameterl = np,  dm = d − p/2
RaiseTR = (Fdm/2)(l + πfdm)/(πdm − fl) + Ffcdc/2
LowerTL = (Fdm/2)(πfdm − l)/(πdm + fl) + Ffcdc/2
Self-lockingπfdm > l (times sec α for Acme)

Worked example

Double square thread, d = 32 mm, p = 4 mm, f = fc = 0.08, dc = 40 mm, F = 6.4 kN (Shigley Example 8-1, the default)

  1. l = 8 mm, dm = 30 mm.
  2. TR = 15.94 + 10.24 = 26.18 N·m; TL = −0.47 + 10.24 = 9.77 N·m.
  3. e = 6400 × 0.008/(2π × 26.18) = 0.31.

Frequently asked questions

Why is the efficiency so low?

Friction on the thread and especially the collar absorbs most of the input work; ball screws replace sliding with rolling to reach about 90%.